HTML5 Agents – Mobile Agents for the Web
Kari Systä, Tommi Mikkonen and Laura Järvenpää
Department of Pervasive Computing, Tampere University of Technology, BOX 553, 33101 Tampere, Finland
Keywords: HTML5, Mobile Agents.
Abstract: The emergence of HTML5 allows more applications to be run in browsers. Although most of these
applications are often network connected, they can also run in off-line mode and especially after
deployment they are not necessarily dependent on any server. We argue that the modern Web infrastructure
with HTML5 as such can be an agent platform and mobile agents could be developed in similar way as Web
applications. For us the agents can also be end-user applications that the user can send to a server so that the
state is preserved and the execution can continue. The user can later fetch the agent to the same client device
or to another device. In addition to the mobile agent use cases, the concept also allows users to continue
their work later on another device or even allows other users to continue execution in their own devices. The
paper presents the overall concept and architecture of HTML5 agents, a number of use cases, the proof-of-
concept implementation, and a list of example applications.
1 INTRODUCTION
HTML5, the latest version of standards in the
HTML family, extends the applicability of the
technology towards client-side applications.
Traditionally, Web applications have more or less
been acting as user interfaces for applications
running in a server, but the goal of HTML5 is to
allow the development of complete client-side
applications. Consequently the emergence of
HTML5 allows more applications to be run in
browser, and these applications can be deployed
over network by using the standard Web
technologies. Moreover, although most of these
applications are often network connected, they can
also run in off-line mode and especially after
deployment they are not necessarily dependent on
any server. For us the overall goal of HTML5 to
support rich applications is important, and in this
paper we do not refer to any specific new technology
introduced by HTML5.
The capabilities of HTML5 described above,
together with other recent developments, such as
increasing pervasiveness of the Web and improving
performance of JavaScript virtual machines inside
browsers, enable the introduction of Web-based
application platforms and operating systems –
something that can be called Web operating systems.
In these systems applications can be stored in a
cache so that downloading is not necessary at the
subsequent invocations of the application. For the
user this means that the applications do not differ
from using traditional installed applications.
Examples of such systems include Cloudberry
(Taivalsaari and Systä, 2012), Google Chromium
OS (http://www.chromium.org/chromium-os) and
Firefox OS of Mozilla (http://www.mozilla.org/en-
US/firefoxos). Of these systems, Chromium OS is
almost a generic operating system for web-enabled
devices, whereas Firefox OS and Cloudberry are
systems where all user-visible functionality of a
smart phone is implemented as HTML5
applications. The listed systems also prove the above
claim that HTML5 technology can be used to build
complete and advanced applications which in many
ways are indistinguishable from traditional,
installable binary applications.
From the technological perspective, however,
Web operating systems differ from traditional
application platforms, because they build on
principles of the Web instead of more traditional
concepts associated with operating systems and
binary applications. Although applications are often
cached, they are not necessarily explicitly “installed”
and updates can be delivered automatically and
without bothering the end user.
The development of Web operating systems and
especially Cloudberry (Taivalsaari and Systä, 2012)
37
Systä K., Mikkonen T. and Järvenpää L..
HTML5 Agents – Mobile Agents for the Web.
DOI: 10.5220/0004368800370044
In Proceedings of the 9th International Conference on Web Information Systems and Technologies (WEBIST-2013), pages 37-44
ISBN: 978-989-8565-54-9
Copyright
c
2013 SCITEPRESS (Science and Technology Publications, Lda.)
have inspired an idea of Cloud browser (Taivalsaari
et al., 2013). In Cloud browser the browser session –
in essence a set of active “pages” and HTML5
applications, both of which are identified by
associated URLs – constitutes the user session. This
user session – the URLs of applications and
information on the state of the applications – is
stored in the Web.
The Web is based on mobile code. Four para-
digms of mobile code have been presented in
(Carzaniga et al., 1997):
Client-Server where client uses code that is located
in another node.
Remote Evaluation where client sends execution
instructions, for example SQL queries, to another
node.
Code On Demand where code is downloaded to
the client for execution. HTML5 applications are
widely used examples of code-on-demand.
Mobile Agent where code together with internal
state of the application is moved to other node for
execution.
The first three paradigms are regularly used in
Web applications and (Taivalsaari et al., 2013)
presented a concept where the internal state of
HTML5 applications were stored in server in the
cloud.
This paper builds on the above ideas, where the
browser is increasingly acting as the application
platform, and where applications can store their
internal state in the server for future use. In addition,
we propose moving the executable code with the
internal state of the application. As a concrete
contribution, we propose a system where the
applications can also continue their execution while
being stored in the server, and the running
applications can later be retrieved back to a browser.
We consider these applications as mobile agents
since they comply with commonly used definitions
of mobile agents, like the ones presented in (Kotz
and Gray, 1999) and (Yu et al., 2006). Although our
current applications do not include autonomous
migration, the proposed approach supports it at the
conceptual level. Our agents are implemented as
HTML5 applications and thus we call them HTML5
agents in this paper.
We claim that HTML5 technology provides
important benefits in implementation and use of
mobile agent for two main reasons:
1. HTML5 technologies are widely used and have a
strong ecosystem. This means that platforms are
widely available and there are de-facto standards and
tools.
2. A new class of use cases for mobile agents
becomes available since the users can run the same
application as a normal application and transfer it to
an agent server, and pull it back in another context.
This puts the users in control and makes the agent
platform more user-oriented.
Many applications can also be implemented with
other paradigms presented in (Carzaniga et al.,
1997). Especially, some of our use cases could be
implemented with Client-Server paradigm so that
most of the logic is moved to server and only the
user interface is in browser. We believe that our
approach is more suitable for cases where:
local execution gives additional value because of
access to local resources, responsiveness
requirements or cost or quality issues in network
quality, and
the original source location of application cannot
be used as a host for execution, e.g., for
commercial or privacy reasons.
In addition, the agents can visit several servers
and browser clients during its execution. This
enables the use cases that typically require
specialized agent platform.
The rest of this paper has been organized as
follows. Section 2 describes the proposed archi-
tecture, and a number of use cases that can be
associated with our approach. Section 3 introduces
our proof-of-concept implementation of the frame-
work, and Section 4 discusses some example
applications. Section 5 discusses related work, and
pinpoints unique features of our approach. Section 6
provides a discussion on the lessons we have learned
in the development process and potential ideas for
future work. Towards the end of the paper, Section 7
draws some final conclusions.
2 ARCHITECTURE AND USE
CASES
In the following, we address two principal elements
of our work. First, we discuss how we have realized
the system at the level of principal design, and then,
we provide some use cases that can be implemented
with our system. We place the focus on conceptual
level, and do not yet go to concrete technical details
of the system, which will be addressed later on in the
paper.
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2.1 Architecture
In the proposed system, an HTML5 agent can run in
two modes:
1. With a user interface inside a client runtime
engine, i.e. the browser.
2. In a headless mode, i.e. without a user interface,
in an application server that we call agent server
in this paper.
The agent can move between these modes and
locations when the browser pulls the agent from a
server and when the browser pushes the agent back
to a server. Furthermore, we support multi-device
usage – the browser instance that pulls an executing
agent can be different from the browser instance that
had originally pushed the agent to the server.
Therefore, during its life-cycle the agent may visit
several browsers and several agent servers. An
example life cycle is presented in Figure 1.
Figure 1: Life-cycle of HTML5 agent.
In Figure 1, agent is started by Browser1, when the
agent is downloaded from its origin server (Step 1).
In this phase the agent is initialized and the
execution begins. Since the agent executes in a
browser it has a user interface. In Step 2, the agent is
pushed to an agent server. This means that the agent
server gets the internal execution state of the agent
and the application code (actually a URL to the
code). The agent can continue the execution in the
server. In Steps 3-5, the agent moves from one
environment to other but preserves its internal state
and continues execution. Finally, the execution is
terminated in Step 6. Note that in life cycle shown in
Figure 1, as well as in our experimental
implementation, too, the agent always moves
between server and client. However, it would be
trivial to make the agent to move at least between
servers.
It should also be noted that not all applications
need to execute in the server, but execution resumes
when the agent migrates to (some) browser again.
For example, in the media player scenario presented
in (Yu et al., 2006) playing should be resumed when
agent is in the browser again. However, for some
systems, headless mode may provide important
functions that are essential for satisfying certain
types of requirements. In case of monitoring
applications, for instance, certain events might
trigger moving the application back to the client so
that the user can take appropriate actions. Naturally,
this would require presence of some push
notification technology.
Internal state of the application is an important
part of a mobile agent. The state needs to be
serialized, transferred and de-serialized in the new
location. It is obvious that without modifying the
implementations of the browser the complete state of
the application cannot be serialized, and the agent
needs to be written so that serialization of the
relevant components of the state is possible. Our
design provides support for such serialization of the
important parts of the state.
In our current implementation a single agent
instance moves from host to host, but it would be
trivial to change the behaviour so that a new copy is
created when needed. For instance when a browser
fetches the agent from the agent server, a new copy
could be created and the original application could
also continue its execution in the agent server. We
believe that this can be performed with configuration
options included in the framework, but this remains
a piece of future work as discussed towards the end
of the paper.
Since the agent can run both with and without
UI, the architecture has to be designed to separate UI
from execution. HTML5 provides a good ground for
this separation. Similarly to most HTML5
applications, HTML5 agents are composed of two
major parts:
1. Declarative description of the user interface in a
form of HTML, CSS and image files.
2. JavaScript-files describing the executable code.
As is common in today’s web applications, the
HTML-file includes references to Cascading Style
Sheets (CSS), images and other resources, and
JavaScript files.
When the HTML5 agent is running in the agent
server, it runs in the headless mode and the HTML
and CSS files are not needed. Only the virtual
machine executing JavaScript is needed for the
execution of the agent. Naturally, whenever the
agent moves to a browser HTML and CSS files need
to be available again. Therefore, the agents need to
preserve at least URLs of the UI components of the
agent. Also the JavaScript code of the agents has to
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39
be written in such a way that it can be executed
without presence of the Document Object Model
(DOM) tree, the data structure that is used as an
internal presentation of a web page inside the
browser. (W3C, 2005) We provide a simple browser
emulation system that supports running of browser-
based applications in a server, but still certain coding
rules need to be followed.
The execution model of the application also
needs to be suitable for our approach. First of all it
needs to be suitable for running in the browser, for
instance it should not block the event loop of the
browser run-time. On the other hand it needs to
execute without UI events delivered by the browser.
Furthermore, the agent needs to have safe points in
execution in which the internal state can be
serialized in consistent state. In practice this means
that all the application logic has to be embedded in
the event handlers and in addition to handlers for UI
events there is a handler for timer events that are
generated by our framework.
If the only requirement is that the application and
its internal state are stored to an agent server but the
application does not need to execute there, there are
fewer constraints for the design. The application
state must be serialized, but support for the agent
execution model – timer-based events – is not
needed.
2.2 Use Cases
Our system targets two kinds of use cases:
traditional use cases of mobile agents and long-
lasting sessions in web applications that are used in
multi-device environment.
We fundamentally agree with (Lange and
Oshima, 1999) and assume no single killer
application for mobile agents, but several
applications can benefit from mobile agent
technology. From the example application areas
presented in (Lange and Oshima, 1999) the
following would at least benefit from our solution:
e-Commerce. If the product a user wants is not
immediately available, or if the pricing is not
satisfactory, the user can set her constraints and then
send the e-commerce application to the agent server
for execution.
Personal Assistance. The example given in (Lange
and Oshima, 1999) ”to schedule a meeting with
several other people, a user can send a mobile
agent to interact with the agents representing each of
the people invited to the meeting. The agents
negotiate and establish a meeting time.” would be
good use case for us, too.
Secure Brokering. The agent server could be a
mutually trusted host that enables collaboration
between the agents.
Workflow Applications. The workflow application
can be a web application that is executed both in a
server and in browsers of several users.
Among web applications all applications that
should execute continuously but the user would still
like to have a break or just switch to another device
are potential use cases of our HTML5 agents. With
normal web applications the application state is lost
when user switches from one device to another.
Our solution allows users to continue their work
in another device. The novel idea presented in this
paper is that the execution of the application can also
continue while user is not using it through any
device. This is beneficial for instance, in the
following cases:
monitoring applications that collect data about
events or values of various sensors,
strategy games that should continue execution of
users instructions as robots when user is not
present, and
applications whose execution takes a lot of time or
need to access big server-side resources are better
executed in the agent server.
Many of these applications can also be
implemented as server-side applications that users
access with a browser. However, the server-side
applications are typically bound to a specific service
and configurability for individual users is limited.
Some applications would also like to access
resources in the client device and thus the execution
should take place in the client device. Finally, the
ability to move computational agents between
clients and servers adds an extra layer of flexibility.
3 PROOF-OF-CONCEPT
IMPLEMENTATION
To test and validate our idea we have created an
experimental implementation of the agent
architecture. The server side of the implementation
is based on Node.js technology (Nodejs, n.d.), which
is a platform for the development of scalable
network applications using JavaScript (Taivalsaari
and Systä, 2012). Thanks to Node.js we can execute
the same JavaScript code both in server and in
browser.
The implementation consists of three main
components: 1) agent framework that acts as a
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superclass for the agent applications, 2) execution
context in the server-side including an emulation of
the browser environment, 3) serialization format and
transfer protocol for application state and
information. More detailed explanations of these
components are given in the following.
3.1 The Agent Framework
The execution model is event-based which is normal
in browsers and Node.js server. To support
execution of possible computing tasks of the agent, a
periodic timer calls an application-specific work-
function on regular intervals.
As explained earlier, the complete internal state
cannot be stored and migrated by the system and
also in this experimental system the application need
to explicitly define state information that should be
serialized and delivered when the agent moves from
a location to another. In practice, the agents have
variable list that contains the state that has been
saved and transferred.
The superclass Agent is implemented by using
the functional inheritance pattern presented in
(Crockford, 2008). In this inheritance pattern the
super class defines the set of methods as follows:
function Agent(src,html) {
var that = {};
that.method1 = function(args) {…}
that.method2 = function(args) {…}
return that;
}
This Agent class is not to be instantiated, but each
concrete agent application inherits from Agent as
follows:
function MyAgent(src,html) {
var that = Agent(src,html);
/* new and overridden methods */
}
In our design, the class Agent offers the following
utility methods:
createVar(name,value) creates a new variable
to the variable list, i.e. extends the state of the
agent.
setVal (name,value) sets a value of a
variable in the state of the agent.
getVal(name) – gets value of a variable in the
state of the agent.
setWork(function, interval) – sets the
function that is periodically executed with the
given interval.
setRunInterval(interval) – changes the
interval between executions.
start() and stop() – starts/resumes and stops
execution of the agent. In practice, the timer is
started and stopped.
serialize() – returns a JSON-string that
includes all necessary information to preserve the
state and continue execution of the agent in a new
location. This method is for the framework and is
not usually called from application code.
The application-specific sub-class of the Agent can
override the following methods:
Method getRunningStatus() should return a
string that the management interface of the agent
server context can show.
Function
continueWork() – re-initializes the
execution when the agent has arrived and de-
serialized in a new location and the execution
should be resumed. This function initializes the
state of the agent by recreating the variables.
In addition, the agent has to provide a function
that creates and initializes the agent object.
3.2 Managing Execution Context
in the Server Side
As hinted earlier we have implemented a simple
HTTP server with Node.js. This server receives
description of the agent, i.e., the location of
executable JavaScript file(s) and state information,
in an HTTP-POST request. Then, the following
steps are performed:
The executable JavaScript file is downloaded from
the origin server.
The required run-time structures are created.
The function
continueWork() of the down-
loaded agent is called.
A timer to periodically fire the work-function is
initialized.
The application code of web applications
typically assumes existence of a DOM tree created
from the HTML. For the headless mode we have a
minimal browser emulation layer for applications
that refer to HTML document. The design principle
is to provide as minimal emulation as possible and
require applications to adapt, too. For example,
some of our example applications draw graphics
through Canvas API (W3C, 2004). The application
code should check if Canvas API is available, in
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41
accordance to the following snippet of code:
var ctx = comp.getContext("2d");
if (ctx != null) {
that.draw(…);
}
This would actually be a good coding practice
for all web applications since
getContext() could
return null in browsers, too. Our emulation
implementation of
getContext() is simply the
following:
this.getContext = function () {
return null;
}
In addition, our browser emulation framework
includes implementation of standard JavaScript
function
getElementById(id) that always returns
a new object that carries the given id and
implements our emulation interface. We also have
toString() that is mainly used for debugging
purposes.
In order to use the same API for network
configuration, we have installed an implementation
of XMLHttpRequest (W3C, 2013) module to our
Node.js server.
3.3 Serialization Format and Transfer
Protocol
In our design, the browser can receive the agent in
two alternative ways:
1. by fetching the HTML-files of the application
from the origin server, or
2. by fetching an agent description from agent
server.
In contrast, the agent server gets the agent by
3. receiving an agent description in a HTTP
POST message.
In cases 2 and 3 information about the agent and
its internal state is encapsulated in an agent
description that is a JSON document containing the
following information:
auri: a URL that points to the JavaScript file of the
application. The agent runtime, in the new
execution location, needs to download the JavaScript
file from this URL.
huri: a URL pointing to HTML file, i.e., UI of the
application. The HTML file then refers to required
CSS and image files.
id: unique ID of the agent instance.
variables: local state in terms of names and values
of local variables.
The utility method serialize() of class Agent
creates this agent description. When the agent server
receives a request to pull an agent (Step 3 in Figure
1) the agent server calls method
serialize() of
running agent and includes the result in the response.
When client browser wants to push agent to the
agent server, it also calls this method and embeds
the result to the HTTP POST request that send the
agent to the server.
For optimization reasons, when the application is
fetched from agent server, the content of the HTML
file is attached to the agent description.
The implementation of our agent system requires
the browser to fetch content from several origins. To
enable this we have used the Cross-Origin Resource
Sharing (CORS) (W3C, 2013) technology to allow
use of several origins.
4 EXAMPLE APPLICATIONS
We have experimented and tested the framework
with some test applications. The example
applications have been used to further develop and
validate different aspects of our agent framework.
Monitoring. Our first example application was a
simple monitoring application that tracks the CPU
load of a server machine. While implementing this
application we also made most of our current design
decisions. When this agent is run in a browser, the
application shows current load level, minimum and
maximum values, and latest history graphically.
When this application is run in a server, the
information is still collected so that a non-
interrupted sequence of data can be shown when the
agent has migrated back to a browser. This
application was tested both on PC browsers and on a
smartphone.
Image Analysis. We also wanted to test the
framework with an application that contains long-
lasting computation and user would like to push the
computation to the server after seeing that it has
been started correctly. As a concrete example we
selected an image analysis application that can
compute characteristics of images both in browser
and server. With this application we investigated
issues with large amounts of data, and the main
learning is that the future agents should have
pointers in the resources in the Internet instead of
carrying the data in the state of the agent.
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On-line query applications. We assume that
many applications of our HTML5 agents collect
information for the user. The motivation could be e-
commerce or just to get information when it
becomes available. To experiment with this
application area we have implemented an agent that
lets users to follow an on-line artist community - in
our case DeviantArt (Deviant art, n.d) for new
content from favourite artists or by selected
keywords. With this application we also
experimented with a computation that had behave
slightly differently in browser and server because the
source of RSS feed provides different format and
content for different clients.
While the above applications demonstrate the
main capabilities of the present implementation, we
will continue development of new applications when
new and improved capabilities are introduced. Some
of these ideas are discussed in Section 6 of this
paper.
5 RELATED WORK
As already pointed out, our agents comply with
commonly used definitions such as “Mobile agents
are programs that can migrate from host to host in a
network, at times and to places of their own
choosing. The state of the running program is saved,
transported to the new host, and re stored, allowing
the program to continue where it left off
(Kotz and Gray, 1999) or “Mobile agents are self-
contained and identifiable computer programs that
can move autonomously within the network and act
on behalf of the user or other entities. A mobile
agent can execute at a host for a while, halt
execution, dispatch itself to another host, and
resume execution there”. (Yu et al., 2006)
A survey of mobile agent technologies has been
given in (Gupta and Kansal, 2011), but the
discussion is limited to traditional mobile agents. In
this paper we propose a new approach in which
HTML5 - a standard feature of a browser - and the
emulation in application server constitute the agent
platform.
Benefits and application areas of mobile agents
have been discussed in (Lange and Oshima, 1999).
From the motivations presented in (Lange and
Oshima, 1999), at least reduction of network load
and latency, asynchronous and anonymous
execution are valid for HTML5 agents, too. Also
many of the presented application areas are common
to HTML5 agents.
An agent platform hosted in browsers has been
presented in (Feldman, 2007). This work has
common targets with ours; it allows agents to be
executed both in server and browser. In (Feldman,
2007) mobile agent platform is based on concepts of
Pneuna that is relatively close to our agent
description and Soma that is the execution
environment. In the approach proposed in (Feldman,
2007) Soma hides the differences of browser and
server environment and creates a completely new
application platform for mobile agents. In our
approach standard and well-known HTML5 is the
agent/application platform. In addition the approach
presented in (Feldman, 2007) has not been designed
for pushing agents to agent server when user or
browser is not active or when the agent should find a
new browser to run on.
The basic idea presented in (Yu et al., 2006) is
somewhat similar to our approach, but they do not
use HTML5 as the application platform. For
example, their media player example would fit
nicely to our approach, too, especially if HTML5
media API would be used to implement the player.
A particularly interesting aspect in (Yu et al., 2006)
is self-adaptation and context awareness – in
practice different UI to different devices. It would be
interesting to implement similar behaviour using
web technologies.
The relation between trends in the Internet and in
mobile agents has also been discussed in (Kotz and
Gray, 1999). The paper discusses these trends and
forecasts that “within a few years, nearly all major
Internet sites will be capable of hosting and willing
to host some form of mobile code or mobile agents.”
This paper has also interesting discussions about
technical and non-technical hurdles of mobile
agents. From the presented technical hurdles using
HTML5 overcomes “standardization and
portability” and to limited extent also “security”.
From the non-technical hurdles our approach solves
“Getting ahead of the evolutionary path” because we
use the de-facto HTML5 technology, and “Revenue
and image” has effectively been eliminated by the
evolution of the Internet and its business models.
6 FUTURE WORK
We are still in initial phase of our work and have
only tested our approach with a limited number of
applications. Although we have proven that the
approach works with these applications, we
anticipate a need to improve the architecture so that
development of new agents becomes easier. We
need to develop new applications and collect
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43
feedback from other developers. For example, our
browser emulation interface for the server side is
still incomplete, and it has only been tested with
very a limited set of test applications. Therefore new
features must and have been added to the system as
new applications are implemented.
Context awareness, as in (Yu et al., 2006) should
also be added, and we should work more with
different types of devices. In particular context
awareness should be complemented with
mechanisms for self-adaptation, which would
provide extra flexibility in some use cases.
Security is an obvious concern for all
dynamically moving code. In the current system we
rely on standard security mechanisms of HTML5
applications in browser. However, if the agents need
to execute sensitive tasks in the server or agents of
several users should collaborate in the server, we
need to develop additional security mechanisms that
are missing from today’s Web.
In our current example a single instance of an
agent migrates between hosts. Extensions to
multiplying agents should be experimented with and
we should develop configuration techniques for
when to move applications and when to multiply
them.
Finally, exploring agents that are able to
autonomously extend their behaviour in accordance
to the needs of the application is one of the
directions we have been considering. Building on
the immense flexibility of the Web and web
applications, this would be a step towards mashware
where components offered as a service form the
basis for constructing applications (Mikkonen and
Salminen, 2012).
7 CONCLUSIONS
Recent development of web technologies is rapidly
gearing the Web towards a role where it offers more
and more facilities that have been commonly
associated with traditional operating systems and
binary applications.
In this paper, we have shown that HTML5
technology can be used to implement mobile agents
and that use of agent approach can improve the user
experience especially in multi-device scenario. In
addition, we introduced a proof-of-concept
implementation that is able to run simple
applications. While as future work, we list a number
of ideas that will improve the capabilities of the
system; we believe that the present implementation
validates the feasibility of the fundamental design.
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