The Lazy Scheme

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The Lazy Scheme


We now present our lazy scheme for providing consistent views for running transactions. Our scheme assumes that server clocks never run backwards, and advance rapidly enough that each transaction can be assigned a distinct timestamp; these assumptions are easy to guarantee (see for example, the discussion in [21]).

The basis of the scheme is the invalidations generated when transactions commit. The fundamental idea is this: if a client running transaction U observes a modification made by transaction T, then it must already have received all the invalidations of T and any transactions T depended on.

The information about invalidations is conveyed to clients using _multistamps_. Each committed transaction has a multistamp that indicates its invalidations and those of all transactions it depends on. A multistamp is a set of tuples <_client, server, timestamp_>; each tuple <_C, S, ts_> means that an invalidation was generated for client C at server S at time _ts_. The timestamp _ts_ is the value of S's clock at the time it prepared a transaction that caused invalidations for C.

We assume the obvious merge operation on multistamps: if the two input multistamps contain a tuple for the same client/server pair, the merge retains the larger timestamp value for that pair.

The next two subsections describe the processing at the server and the client, ignoring size issues: multistamps are allowed to grow without bound and so are local tables at the server. Section 5.3 describes how we solve the size problems. Section 5.4 gives an informal argument that the scheme is correct.

Atul Adya
Wed Jun 25 15:09:14 EDT 1997