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Writing

The $MT \! B$ associated with an open for writing is the "mutating" counterpart of the $CT \! B$ associated with an open for reading. The command is similar:

open(f,WRITING,$MT \! B$)

When a file, f (as in the above command), is opened for writing and $MT \! B$ is the specified time bound, then the following "rules" apply:

• If $MT \! B$$>$0 then f must be and remain consistent within the specified time bound. This is pessimistic writing.
  • Note: In this case consistency should be thought of as: The file is not updated on the server by other clients. The replica, on the client that opens the file for writing, of course do not stay consistent (because it is written to), but it will "merge" successfully with the primary copy if it reaches the server before the lock expires--so in this sense it is consistent.
• If $MT \! B$=0 then f must be consistent (now). This is strict writing.
• If $MT \! B$$<$0 then f is guaranteed to have been consistent no longer ago then specified by the time bound and on last check. This is optimistic writing.
  • Note: Compared to reads the file is additionally required to have been consistent on the last check (i.e., $CT$=$CCT$), for else the writing is bound to fail (it is inconsistent even before it is begone).

As with the $CT \! B$ the $MT \! B$ 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!

The correspondence between the modification time bound and the level of optimism or pessimism when opening a file for writing is depicted in Figure 5.2.

Figure 5.2: Modification Time Bound (MTB)

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

• ANSI-C: FILE *fopen(char *pathname,char *mode);
  • mode $\in$ {"r+","r+b","w","wb","w+","w+b","a","ab", "a+","a+b"}
• $\mathcal{P}$$e$$\mathcal{S}$$t$$\mathcal{O}$: FILE *p_open(char *pathname,char *mode,int mtb);

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