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Replication of Non-Deterministic Objects REPLICATION OF NON-DETERMINISTIC OBJECTS THÈSE NO 1903 (1998) PRÉSENTÉE AU DÉPARTEMENT D'INFORMATIQUE ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Thomas WOLF Dipl. Informatik-Ing. ETH originaire de Thayngen (SH) acceptée sur proposition du jury: Prof. A. Strohmeier, directeur de thèse Prof. H.-H. Naegeli, rapporteur Prof. A. Schiper, rapporteur Prof. A. Wellings, rapporteur Lausanne, EPFL 1998 Abstract This thesis discusses replication of non–deterministic objects in distributed systems to achieve fault tolerance against crash failures. The objects replicated are the virtual nodes of a distributed application. Replication is viewed as an issue that is to be dealt with only dur- ing the configuration of a distributed application and that should not affect the development of the application. Hence, replication of virtual nodes should be transparent to the applica- tion. Like all measures to achieve fault tolerance, replication introduces redundancy in the system. Not surprisingly, the main difficulty is guaranteeing the consistency of all replicas such that they behave in the same way as if the object was not replicated (replication trans- parency). This is further complicated if active objects (like virtual nodes) are replicated, and these objects themselves can be clients of still further objects in the distributed application. The problems of replication of active non–deterministic objects are analyzed in the context of distributed Ada 95 applications. The ISO standard for Ada 95 defines a model for distributed execution based on remote procedure calls (RPC). Virtual nodes in Ada 95 use this as their sole communication paradigm, but they may contain tasks to execute activities concurrently, thus making the execution potentially non–deterministic due to implicit tim- ing dependencies. Such non–determinism cannot be avoided by choosing deterministic tasking policies. I present two different approaches to maintain replica consistency despite this non– determinism. In a first approach, I consider the run–time support of Ada 95 as a black box (except for the part handling remote communications). This corresponds to a non–determin- istic computation model. I show that replication of non–deterministic virtual nodes requires that remote procedure calls are implemented as nested transactions. Unfortunately, effects of failures are not local to the replicas of a virtual node: when a failure occurs, nested remote calls made to other virtual nodes must be undone. Also, using transactional seman- tics for RPCs necessitates a compromise regarding transparency: the application must iden- tify global state for it cannot be determined reliably in an automatic way. Further study reveals that this approach cannot be implemented in a transparent way at all because the consistency criterion of Ada 95 (linearizability) is much weaker than that of transactions (serializability). An execution of remote procedure calls as transactions may thus lead to incompatibilities with the semantics of the programming language. If remotely called sub- programs on a replicated virtual node perform partial operations, i.e., entry calls on global protected objects, deadlocks that cannot be broken can occur in certain cases. Such dead- locks do not occur when the virtual node is not replicated. The transactional semantics of RPCs must therefore be exposed to the application. A second approach is based on a piecewise deterministic computation model, i.e., the execution of a virtual node is seen as a sequence of deterministic state intervals. Whenever a i Abstract non–deterministic event occurs, a new state interval is started. I study replica organization under this computation model (semi–active replication). In this model, all non–determinis- tic decisions are made on one distinguished replica (the leader), while all other replicas (the followers) are forced to follow the same sequence of non–deterministic events. I show that it suffices to synchronize the followers with the leader upon each observable event, i.e., when the leader sends a message to some other virtual node. It is not necessary to synchronize upon each and every non–deterministic event — which would incur a prohibitively high overhead. Non–deterministic events occurring on the leader between observable events are logged and sent to the followers just before the leader executes an observable event. Conse- quently, it is guaranteed that the followers will reach the same state as the leader, and thus the effects of failures remain mostly local to the replicas. A prototype implementation called RAPIDS (Replicated Ada Partitions In Distributed Systems) serves as a proof of concept for this second approach, demonstrating its feasibil- ity. RAPIDS is an Ada 95 implementation of a replication manager for semi–active replica- tion for the GNAT development system for Ada 95. It is entirely contained within the run– time support and hence largely transparent for the application. ii Résumé Cette thèse traite de la duplication d’objets non–déterministes dans des systèmes répartis afin de les rendre tolérants aux pannes, plus particulièrement aux défaillances par arrêt. Les objets dupliqués sont les nœuds virtuels d’une application répartie. La duplication de nœuds virtuels est censée être transparente pour l’application: on considère la duplication comme un mécanisme qui n’intervient qu’au moment de la configuration de l’application et qui ne devrait pas affecter le développement de celle–ci. Comme toutes les mesures ayant comme but la tolérance aux pannes, la duplication introduit de la redondance dans le système. La difficulté principale à résoudre est évidem- ment de garantir la cohérence de tous les duplicata pour que l’ensemble de duplicata se comporte envers les autres objets comme un objet singulier (transparence de duplication). Le problème est encore amplifié si l’on considère la duplication d’objets actifs, qui peuvent eux–mêmes être des clients d’autres objets dans l’application répartie. Dans cette thèse, les problèmes de la duplication d’objets actifs non–deterministes sont étudiés dans le cadre d’applications réparties mises en œuvre avec le langage de pro- grammation Ada 95. Le standard ISO de ce langage définit un modèle pour l’exécution répartie fondé sur des appels à distance (RPC). Les nœuds virtuels en Ada 95 ne communi- quent que par des appels à distance; ils peuvent néanmoins contenir des tâches, ce qui rend leur comportement non–déterministe en raison de certaines dépendances temporelles impli- cites. Ce genre de non–déterminisme ne peut pas être résolu en choisissant des règles d’ordonnancement de tâches déterministes. Cette thèse présente deux approches différentes pour maintenir la cohérence des duplicata malgré ce non–déterminisme. Dans la première, le support d’exécution de Ada 95 est considéré comme une boîte noire, à l’exception de sa partie responsable du traitement de la communication entre les nœuds virtuels. Ceci correspond à un modèle de calcul non– déterministe. Je montre que la duplication de nœuds virtuels non–déterministes nécessite une sémantique de transactions imbriquées pour les appels à distance. Les effets de défaillances ne sont pourtant pas locaux à l’ensemble des duplicata d’un nœud virtuel: lors d’une défaillance, les appels à distance imbriqués faits à d’autres nœuds doivent être annu- lés. La sémantique transactionnelle nécessite également un compromis en ce qui concerne la transparence: l’application doit identifier son état global, car il ne peut pas être identifié avec certitude de manière transparente dans le support d’exécution. Mais le problème le plus sévère est sans doute qu’il est même impossible de mettre en œuvre cette approche d’une manière transparente car le critère de cohérence de Ada 95 (linéarisabilité) est beau- coup moins fort que celui de transactions (sérialisabilité). L’exécution d’appels à distance comme des transactions peut donc conduire à des incompatibilités avec la sémantique du langage de programmation lui-même. Si un sous–programme appelé à distance sur un nœud virtuel dupliqué accomplit des opérations partiels, c.-à-d. des appels d’entrées d’objets pro- iii Résumé tégés globaux, des interblocages qui ne peuvent pas être résolus sont possibles. De tels interblocages ne surviendraient pas si le nœud virtuel n’était pas dupliqué! Il est donc indis- pensable que la nature transactionnelle des appels à distance soit révélée à l’application. La deuxième approche se fonde sur un modèle de calcul à étapes déterministes (piece- wise deterministic computation model) où l’exécution d’un nœud virtuel est modélisée par une séquence d’intervalles déterministes d’états. À chaque événement non–déterministe, un nouvel intervalle d’état commence. J’étudie l’organisation de duplicata dans ce modèle de calcul par le moyen de la duplication semi–active. Dans cette forme d’organisation, toutes les décisions non–déterministes se font sur un duplicata distingué (appelé leader ou meneur1) et tous les autres duplicata (les followers, ou suiveurs) sont forcés de suivre la même séquence d’événements non–déterministes. Je démontre qu’il suffit de synchroniser les suiveurs avec le meneur chaque fois que ce dernier est sur le point d’exécuter un événe- ment observable, c’est–à–dire quand il envoie un message à un autre nœud virtuel. Il n’est pas nécessaire de les synchroniser à chaque événement — ceci entraînerait une baisse de performance prohibitive. Des événements non–déterministes qui surviennent entre deux événements observables sur le meneur peuvent être enregistrés dans un journal qui est envoyé aux suiveurs juste avant que le meneur exécute un événement observable. On garan- tit ainsi que les suiveurs vont atteindre le même état que le meneur. Avec ce procédé, les effets de défaillances restent presque complètement locaux aux duplicata. Une première mise en œuvre, appelée RAPIDS (Replicated Ada Partitions In Distribu- ted Systems), montre la faisabilité de la deuxième approche fondée sur le modèle de calcul à étapes déterministes. RAPIDS est un gestionnaire de duplicata pour la duplication semi– active pour le système de développement Ada 95 GNAT.
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