Based on the logical structure introduced in the last subsection, we designed and implemented the event trace access interface function library POET. The basic idea is to consider the measured event trace a generic abstract data structure. The analysis tools can access the traces only via a uniform and standardized set of generic procedures.
In order to be able to decode the different trace formats, the POET functions use a trace description file called key file (see fig. 3), as discussed in approach (5) above. For efficiency, the key file is in a binary and compact format. It not only describes data formats and representation of the single values, but also includes user-defined (problem-oriented) identifiers for each record field (field names) and the interpretations for the values of record fields. In order to access event record fields efficiently, in POET a field type is assigned to each record field. These are the following basic field types:
Additionally, there are other types of event record fields which are only relevant to the decoding system: First, there are record length fields, which contain the length of the following record field or of the current or previous event record, and checksums. Second, fields containing irrelevant or uninteresting data, such as blank fields, are called filler.
POET provides the following five types of functions for trace initialization, information retrieval and positioning (see fig. 3):
As an illustration, see the C program fragment in fig. 4. It steps once through an event trace stored in the file trc_file and prints for each event record the record number and the problem-oriented interpretation of the event occurred. The program text is not complete; the declarations are missing and there is no error handling (e.g. it should print an error message, when record field "EVENT" does not exist). But it demonstrates how POET can be used to write a program for analyzing event traces.
In order to make the construction of the access key file more user-friendly, we developed the event trace description language TDL which is designed for a problem-oriented description of event traces. The TDL compiler checks the TDL description for syntactic and semantic correctness and transforms it into the corresponding binary key file (see fig. 5). In this way, the initialization of POET can be much faster as there is no need for error checking.
The development of TDL had two principal aims: the first was to make a language available which clearly and naturally reflects the fundamental structure of an event trace. The second was that even a user not familiar with all details of the language should be able to read and understand a given TDL description. Therefore, TDL is largely adapted to the English language. The notation of syntactic elements of the language and the general structure of a TDL description are closely related to similar constructs in the programming languages PASCAL and C. By writing an event trace description in TDL, one provides at the same time a documentation of the performed measurement.
Beyond that, we use a similar approach for filtering event records depending on the values of their record fields. For efficiency, the filtering layer is integrated into POET. There is an additional function to the POET library (get_next_filtered) which can be used to move the current decoding position within the event trace to the next event record which matches the user-specified restrictions given in a so-called filter file. These rules can be specified in the Filter Description Language FDL. Since the FDL compiler does not only read the filter description but also the key file, the problem-oriented identifiers for field names and interpretations can be used for specifying the filter rules. The FDL compiler can also check for syntactical correctness and consistency.
A prototype of the tool TDL/POET was designed and implemented under the operating system UNIX in the programming language C in 1987. A redesign took place in 1989. The now available version 5.3 is much faster and provides more functions than the prototype. For details, see the comprehensive user's guide . The tools enable us to analyze event traces which were recorded by ZM4 or other monitor systems such as network analyzers, logic analyzers, software monitors or even traces generated by simulation tools (e.g. QNAP). POET is an open interface. This means that the user can build his own customized analysis tools using the POET function library.
Using a complex data interface like POET does not automatically mean loosing performance. Using UNIX profiling we measured that our analysis tools spend about 5%of their time in POET functions. If the tool uses graphical output (e.g. X windows), this fraction was even less than 1%. Therefore, the win in performance when substituting the POET functions with access routines optimized for a special trace format is small, and the object-independence of the tool is lost.