Occasionally Connected Computing¶
A data-work arrangement that preserves local reads and writes through intermittent remote reachability, then exchanges and reconciles changes.
Core Idea¶
Occasionally connected computing supports useful work on locally reachable data while remote shared state is intermittently unreachable. A client or accessible replica can read and change that local state, retain accepted mutations, and later exchange and reconcile them with other work. It does not promise that uncached data are available or that conflicts resolve without intervention.[ref-eafb3fef6331][ref-f76c5f9c7aeb]
Coda and Bayou show two unlike ways to meet this contract. Coda uses a partial client disk cache against shared file servers; Bayou uses accessible replicated collections and occasional pairwise communication. Coda has a disconnected mode, while Bayou explicitly permits degrees of connectedness. Neither a unique central store nor a binary mode is required of the whole class.[ref-eafb3fef6331][ref-f76c5f9c7aeb]
Scope of Application¶
In Coda, Venus serves requests for cached files when relevant servers are unreachable. Cache misses fail. It logs local file mutations and validates/reintegrates them when contact returns; a conflicting change can require repair. The same mechanism supports involuntary network/server interruptions and voluntary portable-computer detachment, so a component failure is not always the trigger.[^ref-eafb3fef6331]
In Bayou, a client can read or write through any accessible server replica, including one on a portable host. Room reservations and bibliographic entries may remain tentative. Pairwise exchange later propagates writes, while application-supplied dependency checks and merge procedures address conflicts. Bayou's eventual-agreement design and primary commit are specific guarantees and mechanisms, not an automatic property of every occasionally connected arrangement.[^ref-f76c5f9c7aeb]
Clarity¶
Ask what shared data relation exists, which data and operations remain locally reachable, which mutations are retained, how changes are exchanged later, and how divergent edits are handled. “Works offline” is too vague to establish these roles. Coda's cached-file limitation and Bayou's accessible-replica condition keep the availability claim bounded.[ref-eafb3fef6331][ref-f76c5f9c7aeb]
A static read-only offline cache with no local change and reconciliation path falls outside this entry's admitted read/write scope. That is an editorial boundary, not a theorem from the original papers. An online-only client that blocks every useful operation during remote unreachability also fails the local-work test.[ref-eafb3fef6331][ref-f76c5f9c7aeb]
Manages Complexity¶
The arrangement separates local acceptance of work from global settlement. A Coda edit can be logged locally yet fail reintegration when a conflict is found. A Bayou reservation can be visible as tentative and change when another replica's work arrives. This distinction prevents “saved locally” from being mistaken for “final everywhere.”[ref-eafb3fef6331][ref-f76c5f9c7aeb]
It also separates a partial cache from a full replica. Both can make data locally reachable, but they differ in coverage, authority, consistency and conflict handling. The common roles are local usefulness under intermittent reachability plus later exchange and reconciliation.[ref-eafb3fef6331][ref-f76c5f9c7aeb]
Abstract Reasoning¶
Suppose an offline user edits a file already present in Coda's cache. The edit can be recorded, but a different uncached file may fail to open. After contact resumes, validation may accept the first edit or expose a conflict needing repair. The working interval therefore does not guarantee complete data access or automatic global acceptance.[^ref-eafb3fef6331]
Replace the cache with a Bayou room-schedule replica. A local tentative reservation can be entered while peers are unreachable; later pairwise anti-entropy reveals another booking and the application's conflict rules act. The two systems keep the same abstract sequence of local work and later settlement while changing topology and consistency machinery.[^ref-f76c5f9c7aeb]
Knowledge Transfer¶
The reusable checklist is remote shared state, locally reachable work state, intermittent contact, retained changes, and a later integration path. It helps assess a new service, but the new service must establish its own cache coverage, accepted operations, failure behavior and conflict policy. Neither Coda's replay procedure nor Bayou's anti-entropy can be assumed without evidence.[ref-eafb3fef6331][ref-f76c5f9c7aeb]
Example¶
Coda portable client. A user's shared Unix files reside on remote servers; Venus caches some files on the client disk. While detached, it serves cached data and logs local changes, but misses fail. Upon reconnection, it validates and reintegrates the log, with manual repair possible for conflicts.[^ref-eafb3fef6331]
Bayou room scheduler. Shared reservations reside at multiple replicas. A user connected to one accessible replica can enter a tentative booking while others are unreachable. Pairwise anti-entropy later spreads the write, and dependency checks and application merge logic handle conflicts; tentative status tells users the booking can move.[^ref-f76c5f9c7aeb]
Neighborhood in Abstraction Space¶
Occasionally Connected Computing sits in a sparse region of the domain-specific corpus (97th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.
Family — Digital Resource Formats & Metadata (7 abstractions)
Nearest neighbors
- Format Relation — 0.78
- Second Normal Form — 0.78
- No-Force Policy — 0.76
- Distributed lock manager — 0.76
- Goal Modeling — 0.76
Computed from structural-signature embeddings · 2026-10-08
Not to Be Confused With¶
Local read/write work under intermittent remote reachability does not require one authoritative server, binary online/offline state, every item cached, or automatic conflict resolution. Bayou's eventual consistency is its own guarantee; Coda's replay can require repair. A read-only static cache without a deferred change path is outside this entry's admitted scope, even though it may make pages viewable without a link.[ref-eafb3fef6331][ref-f76c5f9c7aeb]
References¶
[^ref-eafb3fef6331]: James J. Kistler and M. Satyanarayanan, “Disconnected Operation in the Coda File System”, ACM Transactions on Computer Systems 10(1), 1992, pp. 3–25. DOI: 10.1145/121132.121166. Original full paper, especially §2 and §§4.4–4.5. The cached-file limitation, local replay log, reintegration and possible conflict repair are directly described there.
[^ref-f76c5f9c7aeb]: Douglas B. Terry, Marvin M. Theimer, Karin Petersen, Alan J. Demers, Mike J. Spreitzer and Carl H. Hauser, “Managing Update Conflicts in Bayou, a Weakly Connected Replicated Storage System”, ACM SIGOPS Operating Systems Review 29(5), 1995, pp. 172–182. DOI: 10.1145/224057.224070. Original full paper, especially Abstract, §§1–5; §2 describes the room scheduler and bibliographic database, and §5 covers anti-entropy and eventual agreement under Bayou's design.