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3.18 Using packets

Suppose you made changes to your database, and want to send those changes to someone else but for some reason you cannot use netsync. Or maybe you want to extract and inject individual revisions automatically via an external program. In this case, you can convert the information into packets. Packets are a convenient way to represent revisions and other database contents as plain text with wrapped lines – just what you need if you want to send them in the body of an email.

This is a tutorial on how to transfer single revisions between databases by dumping them from one database to a text file and then reading the dump into a second database.

We will create two databases, A and B, then create a few revisions in A, and transfer part of them to B.

First we initialize the databases (we assume you have a key for commits already):

$ mtn -d A db init
$ mtn -d B db init

Now set up a branch in A:

$ mtn -d A setup -b test test

And let’s put some revisions in that branch:

$ cd test/
$ cat > file
$ mtn add file
$ mtn ci -m "One"    You may need to select a key and type a passphrase here
$ cat > file2
file 2 getting in
$ cat > file
$ mtn add file2
$ mtn ci -m "Two"
$ cat > file
$ mtn ci -m "Three"

OK, that’s enough. Let’s see what we have:

$ cd ..
$ mtn -d A automate select i: | mtn -d A automate toposort -@-

Three revisions! Let’s transfer the first one to the database B. First we get the meta-information on that revision:

$ mtn -d A automate get_revision a423db0ad651c74e41ab2529eca6f17513ccf714
format_version "1"

new_manifest [b6dbdbbe0e7f41e44d9b72f9fe29b1f1a4f47f18]

old_revision []

add_dir ""

add_file "file"
 content [8714e0ef31edb00e33683f575274379955b3526c]

OK, one file was added in this revision. We’ll transfer it. Now, ORDER MATTERS! We should transfer:

  1. the key used to sign things
  2. The file data (fdata) and file deltas (fdeltas), if any
  3. The release data (rdata)
  4. The certs

In that order. This is because certs make reference to revision data and keys, and revision data makes reference to file data and file deltas.

mtn -d A pubkey > KEY_PACKETS
mtn -d A automate packet_for_fdata 8714e0ef31edb00e33683f575274379955b3526c > PACKETS
mtn -d A automate packet_for_rdata a423db0ad651c74e41ab2529eca6f17513ccf714 >> PACKETS
mtn -d A automate packets_for_certs a423db0ad651c74e41ab2529eca6f17513ccf714 >> PACKETS

Database B now contains revision a423db0ad651c74e41ab2529eca6f17513ccf714. You may want to check the PACKETS file to see what the packets look like.

Now let’s transfer one more revision:

mtn -d A automate get_revision d14e89582ad9030e1eb62f563c8721be02ca0b65
format_version "1"

new_manifest [48a03530005d46ed9c31c8f83ad96c4fa22b8b28]

old_revision [a423db0ad651c74e41ab2529eca6f17513ccf714]

add_file "file2"
 content [d2178687226560032947c1deacb39d16a16ea5c6]

patch "file"
 from [8714e0ef31edb00e33683f575274379955b3526c]
   to [8b52d96d4fab6c1e56d6364b0a2673f4111b228e]

From what we see, in this revision we have one new file and one patch, so we do the same we did before for them:

mtn -d A automate packet_for_fdata d2178687226560032947c1deacb39d16a16ea5c6 > PACKETS2
mtn -d A automate packet_for_fdelta
8714e0ef31edb00e33683f575274379955b3526c 8b52d96d4fab6c1e56d6364b0a2673f4111b228e >> PACKETS2
mtn -d A automate packet_for_rdata d14e89582ad9030e1eb62f563c8721be02ca0b65 >> PACKETS2
mtn -d A automate packets_for_certs d14e89582ad9030e1eb62f563c8721be02ca0b65 >> PACKETS2
mtn -d B read < PACKETS2

Fine. The two revisions should be in the second database now. Let’s take a look at what’s in each database:

$ mtn -d A automate select i: | mtn -d A automate toposort -@-

$ mtn -d B automate select i: | mtn -d B automate toposort -@-

Good! B has the two first revisions (as expected), and A has all three. We can checkout from B:

$ mtn -d B co -b test test-B
$ ls test-B
file file2  _MTN
$ more test-B/file
$ more test-B/file2
file 2 getting in

And that’s it! The revisions were successfully transferred.

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