# Create file
![afs-cb-0][1]

1. create file on *host d*
 * data version (dv) = 1
 * callback registration
2. read on *hosts a,b,c*
3. fetch-states
4. add callback at fileserver

# Replicate file
![afs-cb-1][2]
hello.c propagates to *hosts a,b,c* from fileserver. Callbacks are registered for a period of time. Callbacks notify a host if a change occurs.

# Modify file
![afs-cb-2][3]
host a modifies the file.

1. open, write, close on *host a*
2. store-data
3. callbacks *b,c,d* are called
 * "break" callbacks once called (i.e. remove them from the table once called)
4. reply *dv=2* for *host a*
 * incremented at the *fileserver*

# Read stale file
![afs-cb-3][4]
read a stale file on *host c*.

1. open, read on *host c*
2. fetch-status
 * returns *dv=2* > local cached *dv=1*
3. fetch-data *dv=2* for *host c*
 * registers callback for the file on *host c*

# Callback expires
![afs-cb-4][5]

1. callbacks *a,c* expire.
2. read file on *host c*
3. fetch-status
 * returns *dv=2* == local cached *dv*
 * registers a new callback for the file on *host c* with *fileserver*
4. new callback. cached data so local data is used.

# Callback failure
![afs-cb-5][6]
a common failure mode: callback to host fails

1. write, close on *host a*
2. store-data
3. callback (fails)
4. store-data reply
 * *fileserver* increments *dv=3*
5. delayed callback (fails until it does not)
6. callback expires (read fails)

[1]: /diagrams/afs-cb-0.png "Create file"
[2]: /diagrams/afs-cb-1.png "Replicate file"
[3]: /diagrams/afs-cb-2.png "Modify file"
[4]: /diagrams/afs-cb-3.png "Read stale file"
[5]: /diagrams/afs-cb-4.png "Callback expires"
[6]: /diagrams/afs-cb-5.png "Callback failure"