Trying to understand md5sum in databasechangelog

Liquibase Development
1

Hi All,

First post on these forums. I’m trying to understand the checksum calculated by Liquibase. The MD5SUM value in the DATABASECHANGELOG table clearly seems to consist of two parts:

n:hash

Where the prefix n is a number or possible a hex nibble, : must be a separator and then the actual checksum hash.

I have studied the source for Liquibase and I’ve hit a force field with CommandBuilder which I haven’t been able to break down and fully grock it’s inner mechanics. I might add that I’m not a Java developer which make things a bit challening to say the least.

When I calculate a MD5 checksum based on input in the format below as a single string it computes to something else so apparently it is not the whole truth:

filepath::id::author

Does anyone have knowledge about how this actually works and can give me any pointers in the right direction? Any idea what that prefix is?

Thanks,

Andy

2

I can’t really tell you how the md5sum is calculated, but the initial digit appears to the “checksum algorithm version” number. See here for more details:

The combination of filepath:id:author is what is required to make a unique changset in the databasechangelog table. The md5sum represents the “checksum” for the actual code in the changeset.

I’m probably not explaining this exactly correct, but maybe some of what I provided will help you.

1 Like
3

Thanks Daryl!

I believe I might be on a slightly older version of liquibase so that makes sense. Mine says 8.

I’d be surprised if there is an excessive amount of voodoo going on in the actual md5 calculation. I tried a few simple variants of input but nothing turned out right. So, instead of finding out by doing crazy amounts of trial and error tests I started looking at the code.

Further investigation led me to

StandardChangeLogHistoryService.java:

        if (!md5sumRS.isEmpty()) {
            String md5sum = md5sumRS.get(0).get("MD5SUM").toString();
            if (!md5sum.startsWith(CheckSum.getCurrentVersion() + ":")) {
                executor.comment("DatabaseChangeLog checksums are an incompatible version.  Setting them to null " +
                    "so they will be updated on next database update");
                databaseChecksumsCompatible = false;
                UpdateStatement updateStatement = new UpdateStatement(database.getLiquibaseCatalogName(),
                        database.getLiquibaseSchemaName(), database.getDatabaseChangeLogTableName())
                        .addNewColumnValue("MD5SUM", null);

                statementsToExecute.add(updateStatement);
            }
        }

I’d say that confirms your find Daryl.

Is there a way to find all the methods for the CheckSum object? I figure there could be a method for generating the CurrentVersion as well and that might provide some more insights if I can find where it is being called.

4

I’m just a Liquibase user and a DBA. I know nothing about java coding. Hopefully someone else will respond to your question.