Transaction Processing Monitors An Overview

1. Transaction Processing MonitorsAn Overview Module 2 COP 6730

2. Overview • A reference architecture of transaction-oriented system • role of a TP monitor within this framework. • services provided by a TP monitor • structure of this system component

3. The Role of TP Monitors Operating systems, communication systems, etc. are usually not designed for the needs of a transaction-oriented environment: • A TP monitor provides • either essential services absent from the host system, or • services the host performed so poorly that a new implementation was required. • The main function of a TP monitor is to integrate other system components to make them work together to support transaction-oriented processing.

4. Characteristics of TRANSACTION-ORINETED PROCESSING (1) Data sharing: Computations read and update databases shared among all users. Repetitive workload: Users do not run arbitrary programs, but rather request the system to execute certain functions out of a predefined set. Mostly simple functions: Consume 105 – 107 instructions and do some 10 disk I/Os. Variable requests: exhibits some statistical regularity, but cannot be preplanned.

5. Characteristics of TRANSACTION-ORINETED PROCESSING (2) Some batch transactions: have the size and duration of typical batch jobs. Many concurrent users: 103 – 106 High availability: Because of the large number of users, the system must be highly reliable and available. System does recovery. Automatic load balancing: The system should deliver high throughput with guaranteed low response time (soft real-time system).

6. Transaction Types Transaction types are distinguished by three categories • Direct vs queued • Simple vs complex • Local vs distributed local/distributed direct/queued Simple/complex

7. Direct vs Queued Transactions Direct: The terminal and the process running the server program (handling the request) are associated with each other. Queued: Transactions are put in a queue and scheduled for processing according to the queuing discipline. Server Program Server Program

8. Simple vs Complex Transactions Simple • Single message: There is a single input message from the terminal; and upon commit, a single output message is delivered. • Short: The number of object it touches is in the tens. Complex • Conversational: It allows for repeated exchange of messages between the user and the application. • Long: The number of objects it touches is in the tens of thousands (batch-like transaction).

9. Local vs Distributed Transactions • Local: Transactions run entirely on the network node where the request originated (centralized processing). • Distributed: In addition to the local node, transactions may also invoke services from other nodes

10. A TAXONOMY OF TRANSACTION EXECUTION Transaction Direct Queued Single Message Conversational Short Long Local Distributed Local Distributed Local Distribute Local Distribute Direct OLTP Complex Online Queued OLTP Long Batch Transaction Transaction Transaction Transaction (e.g., ad hoc queries) OLTP: Online Transaction Processing

11. Transaction Processing Services • Transaction services must provide a programming environment that integrates transaction control in a seamless manner. • The program needs not worry about concurrency, failures, clean-up, and so forth. • As far as data sharing is concerned, applications can use the services provided by a database service.

12. Transaction Processing Services Apart from the technical issue of access to shared data, more system services are required Manage heterogeneity: Local transaction mechanisms in each subsystem are not sufficient to ensure the ACID properties for the whole function. Control communication: Status of communication sessions must also be subject to transaction control (e.g., Transactional RPC) Terminal management: Since the ACID properties must be perceived by the user, sending and receiving the message must be part of the transaction (e.g., Response delivered to user before failure ?) Presentation services: If the terminal uses sophisticate presentation services, then reestablishing the window environment after a crash is also a part of the transaction guarantee. Context management: Storing and Recovering context must be bound to the SoC Start/restart: TP monitor must also handle restart after any failure. By doing so, all the subsystems are brought up in a state that is consistent with respect to the ACID rules

13. Integrated Control Database transaction control is not all there is to transaction processing • Note: • All components integrated by the transaction services must implement a basic set of protocols that enable them to cooperate in transaction processing • Subsystems that support these protocols are called resource managers

14. Server and Sever Class • Typically, a number of services are bunched together in one application. • Server class is a group of processes (servers) that are able to run the code of a given application program. • At run time, a server class is maintained for each application program. Server Class Server Server Server Server Server • Execution of a service request requires the request to be sent to a process (a server) of the right server class – service invocation.

15. One Process Per Terminal • All applications are linked together to form one application program. • At logon, each terminal is given its own process for the entire session (e.g., time-sharing systems) Process 100 Applications Process 100 Applications Process 100 Applications Process 100 Applications

16. One Process Per Terminal • Problem: • Too many capabilities per processor: Each process comes with more capabilities then a terminal needs. • Too many process switches: Process switches are very expensive operations in most operating systems (2,000 – 5,000 instructions) • Limitation: Acceptable only for small systems of less than 200 clients.

17. Only One Terminal Process • All terminals talk to one process which can be the TP monitor process itself. One Process • The TP monitor process receives the function requests and route them to the programs that can service them 100 Applications EXAMPLE: CICS Transaction Server

18. Only One Terminal ProcessAdvantages & Disadvantages • Advantages: • Makes transaction processing simpler. • The TP monitor can check the function requests, schedule them according to its own polices, and so on. • Disadvantages: • Each page fault or other exception in the process will stop the whole TP environment. • Since a single process can employ only one CPU at a time, the TP system can uses only one CPU. • The process is confined within one address space, which can be a serious limitation for large application.

19. Many Servers, One Scheduler • One (data communication) process handles all request and response messages. • There is a group of processes (i.e., a server class) for each application program. • Different applications are fenced off against each other. • The data communication process routes the service request to the appropriates server.

20. Many Servers, One SchedulerAdvantages & Disadvantages • Example: IMS/DC • Advantages: Simplicity! There is one place for scheduling and load control. • Disadvantages: The data communication resource can become a bottleneck.

21. Many Servers, Many Schedulers A number of (functionally identical) data communication processes do the terminal handling • There is a server class for data communication services. • The communication service must multiplex itself among the terminals it is attached to (i.e., multi-threaded process). Many Data Communication Processes Monitor Process Many Application Servers Terminals Presentation Services Application 1 Application n

22. Many Servers, Many Schedulers • The application server classes are set up as in “many servers one scheduler” scenario. • The presentation service process should be multi-threaded to support multiple terminals • Load control, activation/deactivation of processes, etc. must be coordinated by a separate instance, the monitor process • The application server can be simple, single-threaded processes.

23. Many Servers, Many SchedulersAdvantages & Disadvantages Example: Tandem’s Pathway, DEC’s ACMS. Advantage: • The data communication process is no longer a bottleneck. • Expensive process switches can generally be replaced by much cheaper process-internal thread changes. Disadvantage: Load balance become more difficult.

24. Tasks of TP Monitors (1) • Scheduling: Service requests must be mapped to the proper servers. • Server class management: The TP monitor is responsible for setting up the server class. • Recovery: After a crash, the TP monitor is responsible for bring up the TP environment. • It starts all the system processes, • brings up the server classes, and then • passes control to the transaction manager.

25. The Tasks of TP Monitors (2) • Resource administration: Information about the terminals, databases, application programs, users, etc. is kept in a system repository managed by the TP monitor. • Authentication and authorization: Service requests must be cleared by the TP monitor before they are executed. • System operation: The TP monitor must • provide the operators with sufficient information to tune the system, and • inform them about any problems that occur during normal operations.

26. Resource Managers A resource manager is a software subsystem that ties into the TP monitor to provide protected actions on its state.  It must be able to participate in transaction-oriented recovery TRID: used to tag all subsequent messages BEGIN WORK receive (input message) < some SQL > send (statistics menu) to (window w1); COMMIT WORK;

27. Context-Sensitive Scheduling • The completion of a request typically frees the server so that it can be reassigned to another request. • However, there are cases in which a server is reserved for a special user. Example: For chained transactions, the server must be reserved for the “next” transaction, because it may refer to local context variables available only in that server process.

28. Transaction Manager (TM) Once the transaction program has started, TP monitor has little to do with transaction management. The coordination of the resource mangers is done by the transaction manager.

29. Transaction Manager (TM) cont’ We want to separate • the components exercising transaction control (transaction manager) from • those that do transaction-oriented resource scheduling (TP monitor). Reasons: There are transactions that do not come in though the TP monitor. Examples: • Ad hoc query interface of SQL system. • CAD applications run their own terminal environment.

30. Responsibilities of TP Monitors (1) • The TP monitor brings up the resource managers upon startup. • For restart, the TP monitor only has to bring up the resource managers. The actual recovery protocol is completely handled among the resource managers and the transactions manager.

31. Responsibilities of TP Monitors (2) • To dispatch a server for a request, the TP monitor creates a process (or reuse an existing one) and load the code into it. • All the calls among resource managers are so-called transactional remote procedure calls (TRPCs). The mechanisms to handle them are provided by the TP monitor. Example: BEGIN_WORK is a TRPC to the transaction manager.

32. Transactional Remote Procedure Call (TRPC)

33. Remote Procedure Call (RPC) A RPC system enables a client program to communicate with sever programs on different computers by calling procedures in a similar way to the conventional use of procedure calls in high-level language. Client program Server program Server program Computer 2 Computer 1

34. Export/Import Service • Export Procedures: At the RPC level a service may be viewed as a module with an interface that exports a set of procedures appropriate for operating on some data abstraction or resource. Procedure 1 Resource Procedure 2 Export my procedures Procedure 3 Client Server

35. Export/Import Service • Import Procedures: From the perspective of client programs, a service provides the same facilities as a software module – enabling clients to import its procedures. Import a procedure Procedure 1 Resource Procedure 2 Procedure 3 Client Server

36. Marshalling • Marshalling is the process of taking a collection of data items and assembling them into a form suitable for transmission in a message. • Flatten structured data items into a sequence of basic data items. • Translate those data items into an external data representation. Marshalling

37. Unmarshalling • Unmarshalling is the process of disassembling them on arrival to produce an equivalent collection of data items at the destination. • Translate the external data representation to the local one. • Unflatten the data item. Marshalling Unmarshalling

38. Message Destinations • Potential clients need to know an identifier for communicating with a server. • In the Internet protocols, the destination addresses for messages are specified as • a port number used by a process and • the Internet address of the computer on which it runs. Receive (p, message) Send (p, message) port p port q Message Internet Address

39. RPC: Main Tasks The software that supports remote procedure calling has three main tasks: • Binding: Locating an appropriate server for a particular service. • Interface processing: Integrating the RPC mechanism with client and server programs in convention programming languages. • dispatching of request messages to the appropriate procedure in the server. • marshalling and unmarshalling of arguments in the client and the server. • Communication handling: Transmitting and receiving request and reply messages.

40. Stub Procedure Client computer Server computer Server process Client process Unmarshall arguments Marshall arguments Receive Request Send request Local call Execute procedure Select procedure Return Local return Receive Reply Send Reply Marshall results Unmarshall results Service procedure Client stub procedure Communication module Server stub Client Communication module Dispatcher An RPC system provides a stub procedure to stand in for each remote procedure that is called by the client program.

41. Client Stub Procedure The purpose of a client stub procedure is to convert a local procedure call to a remote procedure call to the server. • marshal the arguments and to pack them up with the procedure identifier into message, • send the message to the server and then await the reply message, • unmarshal it and return the results. Client computer Client process Marshall arguments Send request Local call Local return Receive Reply Unmarshall results Client stub procedure Communication module Client

42. Server Stub Procedure An RPC system provides a despatcher and a set of server stub procedures. Despatcher: uses the procedure identifier in the request message to select one of the server stub procedures and pass on the arguments. Server stub procedure: • unmarshals the arguments, • calls the appropriate service procedure, and • when it returns, marshals the output arguments into a reply message. Server computer Service procedure Server process Unmarshall arguments Receive Request Execute procedure Select procedure Return Send Reply Marshall results Server stub Dispatcher

43. Remote Procedure Calls (RPCs) CALLEE (server) CALLER (client) : Procedure Call : • RPC makes the invocation of services at remote nodes look like local subroutine calls. • The RPC stub on the callee acts fully complementary to the stub at the caller’s side. RPC stub 1. Subroutine Call 3. Subroutine Call RPC stub 2. Request massage Service Routine

44. Interface Definition • The types of the arguments and results in the client stub must conform to those expected by the server stub. This is achieved by the use of a common interface definition. • An RPC interface definition specifies those characteristics of the procedures provided by a server that are visible to the server’s clients: • names of the procedures, and • types of their parameters.

45. Interface Compilers Interface Definition (in Interface Definition Language) Client computer Server computer Server process Client process INTERFACE COMPILER B INTERFACE COMPILER A Unmarshall arguments Marshall arguments Receive Request Send request Local call Execute procedure Client program Server program Dispatcher Select procedure Client stub Server stub Return Local return Receive Reply Send Reply Marshall results Unmarshall results COMPILER B COMPILER B COMPILER A COMPILER B COMPILER A LINKER LINKER SERVER CLIENT RPC Interface compilerscan be designed to process interfaces for use with different languages enabling clients and servers written in different languages to communicate by using RPCs.

46. Interface Compilers Interface Definition (in Interface Definition Language) INTERFACE COMPILER B INTERFACE COMPILER A Client program Server program Dispatcher Client stub Server stub COMPILER B COMPILER B COMPILER A COMPILER B COMPILER A LINKER LINKER SERVER CLIENT RPC Interface compilerscan be designed to process interfaces for use with different languages enabling clients and servers written in different languages to communicate by using RPCs.

47. Invocation of SQL Resource Manager(SQL Pre-compiler) • Server Side: SQL pre-compiler parses and translates the SQL statement into an internal representation that can be interpreted directly by the SQL executor. • Client Side: The pre-compiler also generates code for the host language to call the SQL server: !sqlselect(‘fastsql’, format_CB, expression_CB, &variable_CB); Precompiler Query Executor SELECT … FROM … WHERE … Resource manager name (RMNAME) A resource manager invocation (recognized by the stub compiler) Parameters Entry point

48. Invocation of SQL Resource Manager(SQL Pre-compiler) • Server Side: SQL pre-compiler parses and translates the SQL statement into an internal representation that can be interpreted directly by the SQL executor. • Client Side: The pre-compiler also generates code for the host language to call the SQL server: !sqlselect(‘fastsql’, format_CB, expression_CB, &variable_CB); Resource manager name (RMNAME) A resource manager invocation (recognized by the stub compiler) Parameters Entry point

49. Execution Plans Embedded SQL is compiled once, and from then on the generated query plan is executed. • At compile time, the client has to issue rmCall to the SQL server for it to compile the statement. • The SQL server compiles the statement and generates the access plan, and hands back an ID for that plan. • At run time, the rmCalls from the client refer to the access plan ID and thereby ask the server to run that pre-compiled query.

50. Binding An interface definition specifies a textual service name for a server. However, client request message must be addressed to a server port. Binder (Name Service) Service Name Server Port Register service name & server port Look up service @ this server port SERVER CLIENT