Commit Protocols

 

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Introduction

In distributed data base and transaction systems a distributed commit protocol is required to ensure that the effects of a distributed transaction are atomic, that is, either all the effects of the transaction persist or none persist, whether or not failures occur. Several commit protocols have been proposed in the literature. These are variations of what has become a standard and known as the two-phase commit (2PC) protocol. 

Related Publications:

Apart from other specific publications certain contributions on commit protocols are presented in invited chapters: 

Chrysanthis, P., Samaras, G., and Y. Al-Houmail "Recovery and Performance of Atomic Commit Protocols in Distributed and Multidatabase Systems",  (Invited chapter in) V. Kumar and M. Hsu (Eds), Performance of Database Recovery Mechanism, forthcoming 1997

Samaras G., K. Britton,  A. Citron,  C. Mohan, "Two-Phase Commit Optimizations and Trade offs in the Commercial Environment",  selected to be included in Advances in Concurrency Control and Transaction Processing by K. Ramamritham and P.K. Chrysanthis, IEEE Computer Science Press, ISBN 0-1886-7405-9, 1996

  Samaras, G., and P. Chrysanthis "Commit Protocols", A chapter in: The Encyclopedia of Distributed Computing, P. Dasgupta and J. Urban, Editors, Kluwer Academic Publishers, forthcoming 2000.

Samaras G., K. Britton,  A. Citron,  C. Mohan, "Enhancing SNA's LU6.2 Sync Point  to Include Presumed Abort Protocol", TR# 29.1751. Embedded as chapter 5 in IBM's book publication "Systems Network Architecture, Sync Point Services Architecture", SC31-8134-00, Sept. 1994.

Samaras G., K. Britton,  A. Citron, "Systems Network Architecture, Sync Point Services Architecture", IBM Book, SC31-8134-00, September 1994. Presents IBM's distributed transaction processing architecture. Published and distributed by IBM. This architecture is being implemented by IBM's VM, DB2, CICS, AS400, Transarc's Encina, Tandem and Bull.

 

 Optimization of Commit Protocols

Much of the literature focuses on improving performance in failure cases by providing a non-blocking 2PC that streamlines recovery processing at the expense of extra processing in the normal case.  We focused on improving performance in the normal case based on two assumptions: first, that networks and systems are becoming increasingly reliable, and second, that the need to support high-volume transactions requires a streamlined protocol for the normal case. Our work resulted in a number of optimizations most of which have been incorporated in IBM and non-IBM transactional offerings. These optimizations were presented and analyzed in terms of reliability, savings in log writes and network traffic, and reduction in resource lock time.  Our work's unique contributions include the description of some optimizations not described elsewhere in the literature and a systematic comparison of the optimizations and the environments where they cause the most benefit. Furthermore, it analyzed the feasibility and performance of several combinations of the optimizations and identifies situations where optimizations can be combined effectively. Optimizing for the non-failure case has been, also, demonstrated through this work as the correct approach towards commit optimization. These results have been published in the refered following publications and have significantly influence further work in commit protocols.

Related Publications:

Samaras G., K. Britton,  A. Citron, "Systems Network Architecture, Sync Point Services Architecture", SC31-8134-00, September 1994. Presents IBM's distributed transaction processing architecture. Published and distributed by IBM. This architecture is being implemented by IBM's VM, DB2, CICS, AS400, Transarc's Encina, Tandem and Bull

Samaras G., K. Britton,  A. Citron,  C. Mohan , "Two-Phase Commit Optimizations in a Commercial  Distributed Environment", Distributed and Parallel Databases Journal, 3(4): 325-361, October, 1995

Samaras G., K. Britton, A. Citron, C. Mohan, "Two-Phase Commit Optimizations and Trade offs in the Commercial Environment",   Proc. 9th  International Data Engineering Conference, IEEE, Vienna, April 1993.

Samaras, G., Kyrou G., P. Chrysanthis, "Structuring the Commit Tree to Improve Commit Processing". Proc. 9th Hellenic Conference on Informatics, Nicosia, Cyprus, November 2001.  

 

 Heuristic Decisions and Commit Protocols

Optimizing for the non-failure case does not mean that the ramifications on commit processing due to failure is not of any interest. The reliability aspect of commit processing is extremely important and we could not considered our work fundamentally complete without studying the impact of commit failures in various environments. Heuristic decisions (HD) and their effect on 2PC reliability have been little addressed in the literature but they are considered a practical necessity in the commercial environment.  This part of my work argued the importance of incorporating heuristics decisions in commit protocols and studied the effect of doing so. It provided a comprehensive study of  heuristics decisions: how and when they occur, how they are process, and the importance of being identified and effectively reported to the transaction or commit initiator.  It studied how commit protocols are affected and proposed a 2PC protocol variant, called Presumed Nothing (PrN), that effectively handles heuristic decisions. It described how certain commit optimizations can be used to tune the variant's performance around application requirements, and presented an appropriate commit programming interface. Finally, we studied the effect of adding HD to the main commit variations, such as PA, PC and IBM-PN showing that all collapse (almost) to the PrN.

Related Publications:

Samaras, G., S.D. Nikolopoulos, "Algorithmic Techniques Incorporating Heuristic Decisions in Commit Protocols". Proc. 25th Euromicro Conference, IEEE/CS, Como, September  1995.

Samaras, G. and S. D. Nikolopoulos, "Heuristic Decisions and Commit Protocols", TR# 96-17, University of Cyprus, Computer Science Department.  

Mohan, C., K. Britton,  A. Citron, G. Samaras , "Generalized Presumed Abort: Marrying Presumed Abort and SNA's LU6.2 Commit Protocols', An Iternational Workashop on Advance Transaction Models and Architectures (ATMA96) (Edited Book), Goa, India, September 1996 (Held in conjuction with VLDB'96). Extended version of the HPTS'93 paper.

 

Integration of Commit Protocol Variants

While commit protocol variants (i.e., PA, PC, IBM-PN, PrN, EP, CL, etc.) provide certain efficiencies, when forced in cooperation for the commitment of a distributed transaction in heterogeneous systems, problems and incompatibilities arise.  These incompatibilities appear because of the differences in the commit protocol variants each one of the participants uses. Even if the different participants support the same commit variant, problems might again exist if these variants support  different  commit optimizations. Work in the first three following publications represents the first ever effort in integrating commit protocols. Specifically, the first two, describe the integration of  PA and IBM-PN since these are the dominant commit protocols in use. The resulted protocol, called generalized presumed abort (GPA), has already being implemented in IBM's DB2 V.4, AS400 and CICS.

In our recent work we studied how the different commit protocol variants, enhanced with optimizations  or not, can coexist in the form of a universal commit protocol that allows the execution of distributed transactions in heterogeneous systems. This universal commit protocols is called "All Two-phase Commit"  (ALL2PC). This work resulted in a BS thesis (the last publication below) at the University of Cyprus and is in preparation for conference and journal publication.

Related Publications:

Samaras G., K. Britton,  A. Citron,  C. Mohan , "Enhancing SNA's LU6.2 Sync Point  to Include Presumed Abort Protocol", IBM Technical Report, TR# 29.1751, IBM Research Triangle Park, August 1993.. Embedded as chapter 5 in IBM's book publication "Systems Network Architecture, Sync Point Services Architecture", SC31-8134-00, Sept., 1994.

Mohan, C., K. Britton,  A. Citron, G. Samaras , "Generalized Presumed Abort: Marrying Presumed Abort and SNA's LU6.2 Commit Protocols',  Proc. 4th International Workshop on High Performance Transaction Systems (HPTS), Asilomar, September 1993

Mohan, C., K. Britton,  A. Citron, G. Samaras , "Generalized Presumed Abort: Marrying Presumed Abort and SNA's LU6.2 Commit Protocols', An Iternational Workashop on Advance Transaction Models and Architectures (ATMA96) (Edited Book), Goa, India, September 1996 (Held in conjuction with VLDB'96). Extended version of the HPTS'93 paper.

Andreou Andreas, "Integration of Commit protocols", BS thesis, Supervisor: George Samaras . Dept. of  Computer Science University of Cyprus, May, 1996

 

Adaptability of Commit Protocols

Commit protocols over the years have been extensively optimized to improve performance and system throughput in terms of reliability, savings in log writes and network traffic, and reduction in resource lock time. Every proposed optimization, however, aimed to improve the protocols performance without ever taking into consideration application semantics and needs. This resulted in protocols that could satisfy with good performance only part of applications needs while other requirements could not even be supported. Work in the second publication below provides the ability to adapt commit processing around application semantics. It presents an application requirements classification and commit optimizations that can be used to tune the different variant's performance around application needs, and describes an appropriate commit programming interface. A performance evaluation demonstrates the superior performance  that can be achieved if application needs are considered in the choice of commit processing. The motivation and need for this work came about from the work on "Integration of Commit Protocol Variants" and "Heuristic Decisions and Commit Protocols".  The final goal of this work is to tie together adaptability and integration under one scope.

Related Publications:

Samaras G., K. Britton,  A. Citron,  C. Mohan , "Two-Phase Commit Optimizations in a Commercial  Distributed Environment", Distributed and Parallel Databases Journal, 3(4): 325-361, October, 1995.

Samaras, G., On the Semantic Adaptability of Commit Protocols", technical report TR-98-6, University of Cyprus, April 1998.