Contents   Index

The Consistency Time Bound

The correspondence between the consistency time bound and the level of optimism or pessimism when opening a file for reading is depicted in Figure 5.1.

Figure 5.1: Consistency Time Bound (CTB)

It should be noted that the higher the level of optimism by one client the lower the level of availability for other clients!

To sum up the previous sections:

When a file, f, is opened for reading and $CT \! B$ is the specified consistency time bound, then the following "rules" apply:

• If $CT \! B$$>$0 then f must be and remain consistent within the specified time bound. This is pessimistic reading. With an increasing $CT \! B$ the level of pessimism increases.
• If $CT \! B$=0 then f must be consistent (now). This is strict reading.
• If $CT \! B$$<$0 then f is guaranteed to have been consistent no longer ago than specified by the time bound. This is optimistic reading. With a decreasing $CT \! B$ there is an increasing chance of reading stale data (inconsistent files).

$CT \! B$ should be thought of as a system guarantee, and the user or the application may end up doing better--e.g., if available bandwidth is used intelligently!

Even though we previously assumed that the mobile clients did not stay disconnected forever (see Section 1.3.1), this is in fact possible. All cached files can be read at any time; using an infinitely negative $CT \! B$!

The primitive for open for reading is the ANSI-C [21] fopen with an added parameter for specifying the consistency time bound:

• ANSI-C: FILE *fopen(char *pathname,char *mode);
  • mode $\in$ {"r","rb"}
• $\mathcal{P}$$e$$\mathcal{S}$$t$$\mathcal{O}$: FILE *p_open(char *pathname,char *mode,int ctb);

  Contents   Index