Simulation and performance analysis of distributed Internet systems using TCPNs.(timed coloured cetri net)(Report)

This paper presents a Timed Coloured Petri Nets based programming tool that supports modeling and performance analysis of distributed World Wide Web environments. A distributed Internet system model, initially described in compliance with Queueing Theory (QT) rules, is mapped onto the Timed Coloured Petri Net (TCPN) structure by means of queueing system templates. Then, it is executed and analyzed. The proposed distributed Internet systems modeling and design methodology has been applied for evaluation of several system architectures under different external loads.

Keywords: distributed systems modeling, performance analysis, timed coloured Petri nets

Povzetek: Predstavljeno je orodje na osnovi Petri mrez za modeliranje spletnih okolij.

1 Introduction

One of modern Internet (or Web) systems development approaches assumes that the systems consist of a set of distributed nodes. Dedicated groups of nodes are organized in layers (clusters) conducting predefined services (e.g. WWW service or data base service) [2, 6, 8]. This approach makes it possible to easily scale the system. Additionally, a distributed structure of the system assures its higher dependability. Fig. 1 shows an example cluster-based Internet system structure. The Internet requests are generated by the clients. Then they are distributed by the load balancer among set of computers that constitute the front-end or WWW cluster. The front-end cluster offers a system interface and some procedures that optimize the load of the next system layer-the database servers cluster. In the standard scenario the client produces the request by e.g. filling out the formula on the web side. Then the request is converted into e.g. a SQL query and forwarded to the database layer. The result of the query is sent back to the front-end layer. Finally, the client receives the result of his request on the website.

Simultaneously, for a significant number of Internet applications some kind of soft real-time constraints are formulated. The applications should provide up-to-date data in set time frames [20]. Stock market or multimedia applications may be good examples of hardware/software systems that may have such timing requirements.

The appearing of new above mentioned development paradigms cause that searching for a new method of modeling and timing performance evaluation of distributed Internet systems seems to be an up-to-date research path.

One of intensively investigated branch of Internet systems software engineering is formal languages application for modeling and performance analysis. Amid suggested solutions there are: algebraic description [11], mapping through Queueing Nets (QNs) [8, 18], modeling using both Coloured Petri Nets (CPNs) [12] and Queueing Petri Nets (QPNs) [6, 7].

[FIGURE 1 OMITTED]

Our approach proposed in this paper (1) may be treated as extension of selected solutions summed up in [6, 7], where Queueing Petri Nets (QPNs) language [1] has been successively applied to the web-cluster modeling and performance evaluation. The final QPNs based model can be executed and used for modeled system performance prediction. In our solution we propose alternative Queueing Systems models defined as in [3] expressed into Timed Coloured Petri Nets (TCPNs) [4]. The models has been used as a background for developing a programming tool which is able to map timed behavior of queueing nets by means of simulation. Subsequently we developed our individual TCPNs-based method of modeling and analysis of distributed Internet systems. The well known software toolkits as Design/CPN or CPN Tools can be naturally used for our models simulation and performance analysis [10, 19]. The preliminary version of our software tool was announced in [13], the more mature its description can be found in [16].

The remaining work is organized as follows. In section 2 we informally introduce basic concepts of TCPNs and then in section 3 we provide rules of mapping queueing systems into TCPNs. In the next section, we present a method of applying the TCPNs based queueing systems models (TCPNs templates) to distributed Internet system modeling. Section 5 focuses on results of simulation some detailed Internet system models while section 6 sums up the paper and includes our future research plans.

2 Hierarchical Timed Coloured Petri Nets' Basic Concepts

As TCPNs is the main formal language exploited in the paper we decided to briefly introduce it. The introduction will have an informal form focusing only on the most important TCPNs features. The more thorough TCPNs informal introductions can be found in [9, 5]. The detailed TCPNs features and some applications are presented in [4].

Informally, a Timed Coloured Petri Net is a bipartite graph consisting of "place" nodes and "transition" nodes. The places, drawn as circles or ellipses, are used to represent conditions; the transitions, drawn as bars, are used to represent events.

The places can have some "tokens" associated with. Each token is equipped with an attached data value--the token "colour". The data value may be freely complex (e.g. integer number or record of data). For each place, a colour set is defined which characterizes acceptable colours of the token in the place. All declarations concerning the behavior of the net and colours of tokens are written in the CPN ML language. It is possible to define colours, variables, expressions and functions connected to the elements of the net. The distribution of tokens in the places is called marking. The initial marking determines the initial state of the net and is specified by "initialization expressions". The marking of each place is a multi-set over the colour set attached to the place (compare [4]).

Directed arcs (arrows) connect the places and transitions, with some arcs directed from the places to the transitions and other arcs directed from the transitions to …