I have a python script that supports an --edit-the-database option to invoke the user's preferred editor on a dump of the script's SQLite database. This option is intended to facilitate quick access to parts of the database that the script's other options don't provide access to, particularly during the development of this script.
Once the script has dumped the database's content, launched the editor and verified that the modified content is still valid then it needs to replace the existing database content.
First it removes all existing content by executing this SQL (using python's sqlite module):
PRAGMA writable_schema = 1;
DELETE FROM sqlite_master WHERE type IN ('table', 'index', 'trigger');
PRAGMA writable_schema = 0;
VACUUM;
and then it loads the new content using the sqlite module's executescript() method:
cursor.executescript(sql_slurped_from_user_modified_dump)
The problem is that these two operations (deleting existing content, loading new content) are not executed atomically: press CTRL-C at the wrong moment and the database content has been lost.
If I try to execute those two blocks of code inside a transaction then I get the error:
Error: cannot VACUUM from within a transaction
And if I keep the transaction but remove the VACUUM then I get the error:
Error: table first_table already exists
I have an ugly workaround in place: prior to calling the editor, the script copies the dump file to a safe location, writes a warning message to the user:
WARNING: if anything goes wrong then a backup of the database
can be found in /some/path
and, if the script continues and completes loading the new content, then it deletes the copy of the dump. But this is pretty ugly!
I could use DROP TABLE instead of the DELETE FROM sqlite_master ..., but if I am trying to allow the database to be modified in this way then I am allowing that the list of tables itself may change. I.e. if the user adds this to the dump:
CREATE TABLE t3 (n INT);
then a hard-coded list of DROPs like this:
BEGIN TRANSACTION
DROP TABLE t1;
DROP TABLE t2;
DROP INDEX ...
...
cursor.executescript(sql_slurped_from_user_modified_dump)
...
END TRANSACTION;
isn't going to work second time round (because it doesn't delete table t3).
I could use filesystem-atomic operations (i.e. something like: load the modified dump into a new database file; hardlink new file to old file), but that would require the script to close its database connection and reopen it afterwards, which, for reasons beyond the scope of this question, I would prefer not to do.
Does anybody have any better ideas for atomically replacing the entire content of a database whose list of tables is not predictable?
In case Google leads you here ...
I managed to do the first half of the task (delete existing content inside a single transaction) with something like this pseudocode:
-- Make the order in which tables are dropped irrelevant. Unfortunately, this
-- cannot be done just around the table dropping because it doesn't work inside
-- transactions.
PRAGMA foreign_keys = 0;
BEGIN TRANSACTION;
indexes = (SELECT name
FROM sqlite_master
WHERE type = 'index' AND
name NOT LIKE 'sqlite_autoindex_%';)
triggers = (SELECT name
FROM sqlite_master
WHERE type = 'trigger';)
tables = (SELECT name
FROM sqlite_master
WHERE type = 'table';)
for thing in indexes+triggers+tables:
DROP thing;
At which point I thought the second half (loading new content in the same transaction) would just be this:
cursor.executescript(sql_slurped_from_user_modified_dump)
END TRANSACTION;
-- Reinstate foreign key constraints.
PRAGMA foreign_keys = 1;
Unfortunately, pressing CTRL-C in the middle of the two blocks resulted in any empty database. The cause? cursor.executescript() does an immediate COMMIT before running the provided SQL. That turns the above code into two transactions!
This isn't the first time I've been caught out by this module's hidden transaction
management, but this time I was motivated to try the apsw module instead. This switch was remarkably easy. The latter half of the code now looks like this:
cursor.execute(sql_slurped_from_user_modified_dump)
END TRANSACTION;
-- Reinstate foreign key constraints.
PRAGMA foreign_keys = 1;
and it works perfectly!
The Situation
I recently started working on a new project using nodejs. I have a background of using Python/Django and C#/.NET (not a huge fan of the latter). Node is awesome, but I must say I miss the ease of building models and automating migrations in Django. I am currently using the AdonisJS framework which leverages Knex. Knex is a powerful library, but the migrations all need to be manually built. Additionally, the AdonisJS ORM that manages the Models is independent of Knex (migration manager). You also do not define field attributes on the Models, which can have benifits for dynamically doing things in the front and back end. All things considered, there is a lot of room for human error, miscommunication and a boat load more typing required. I know the the hot thing these days is to keep it loose and fast, but for this specific project, I am looking for a bit more structure than loosely defined models.
Current State
What I have landed on is building a new Class called tableModel and a field class to define the fields within table model. I have already completed this and I am successfully writing the migration files leveraging mustache. I plan on also automatically writing the Models which I shouldn't have a problem with (fingers crossed).
The Problem
Here is where it gets a little tough and where I need help...I need to track what has been added or removed via migration so I can effectively write ups and downs as the tableModels change over time.
So let's say I add a "tableModel" which creates a migration to create table Foo with fields {id (bigint), user_id(int), name(string255)}
Later I want to add a field called description so I would simply add it to my "tableModel" and then run a build command which would build out the migration.
How do I check what has already been created though so I only do an up() for description?
Then I want to remove the name field so I mark it out in my "tableModel" and run a build migration command. How do I check what has been migrated that now needs to be added in to the down().
Edit: I would add a remove field to the up and the corresponding roll back to the down.
Bonus Round
Let's say I want to change user_id from an int to a bigint, because who makes a foreign key just an int? How do I check not just what needs to be added to the up and down, but also checks if I need to change a property on a field.
Edit: would just write the up. and a corresponding roll back to the down
The Big Question
Basically, how do I define dirty "tableModels" classes
Possible Solution?
I am thinking that maybe I should capture some type of registry or snapshot and then run the comparison when building the migrations and or models, then recapture/snapshot. If this is the route, should I store in a json file, write this to the DB itself, or is there another/better option.
If I create the tableModel instances as constants, could I actually write back to the JS file and capture the snapshot as an attribute? IF this is an option, is Node's file system the way to go and what's the best way to do this? Node keep suprising me so I wouldn't be baffled if any of these are an option.
Help!
If anyone has gone down this path before or knows of any tools I could leverage, I would greatly appreciate it and thank you in advance. Also, if I am headed in a completely wrong direction, then please let me know, I both handle and appreciate all types of feedback.
Example
Something to note, when I define the "tableModel" for a given migration or model, it is an instance of the class, I am not creating an extended class since this is not my orm.
class tableModel {
constructor(tableName, modelName = tableName, fields = []) {
this.tableName = tableName
this.modelName = modelName
this.fields = fields
}
// Bunch of other stuff
}
fooTableModel = new tableModel('fooTable', 'fooModel', fields = [
new tableField.stringField('title'),
new tableField.bigIntField('related_user_id'),
new tableField.textField('description','Testing Default',false,true)
]
)
which equates to:
tableModel {
tableName: 'fooTable',
modelName: 'fooModel',
fields:
[ stringField {
name: 'title',
type: 'string',
_unique: false,
allow_null: null,
fieldAttributes: {},
default_value: null },
bigIntField {
name: 'related_user_id',
type: 'bigInteger',
_unique: false,
allow_null: null,
fieldAttributes: {},
default_value: 0 },
textField {
name: 'description',
type: 'text',
_unique: false,
allow_null: true,
fieldAttributes: {},
default_value: 'Testing Default' } ]
You have the up and down notation mixed up. Those are for migrating the "latest" (runs the up function) and doing rollbacks (runs the down function). Up and down to not relate to dropping or adding table columns.
The migrations up is for any change, and the down is to reverse those changes. So if you wanted to drop a column from some table, you write the command in the up, then write the opposite in the down (you'd add it back in...), such that you can "rollback" and the change is effectively reversed. You have to be careful with such things though, as you can put yourself in a situation where you actually lose data.
Want to add a column? Write it in the up, and drop the column in the down.
One of the major points behind the migrations mechanism is to track the state of changes of your database, as time goes forward. So generally, if you created a table in some migration, then a day or so later you realize you need to drop/add columns, you normally don't go back and edit the existing migration, especially if the migration has already been run. You'd just write a new migration to drop/add your column.
Since you're using knex, there are a couple "knex" tables that get created. By default the one you're looking for is knex_migrations, unless someone specifically modified the settings to change the name of it. This table holds all the migrations that have run against your DB, per batch. From the CLI, assuming you have knex.js installed globally, you can run knex migrate:latest, and that will push all the migrations that exist in your directory to the target database, if they have not yet been run. It does this by way of examining that knex_migrations table. If you roll a change and don't like it, and assuming you've properly done the down function, you can invoke knex migrate:rollback to reverse the change. If there are 3 migration files that have NOT yet been run, invoking knex migrate:latest will run all 3 of those migration files under a new batch #, which is 1 higher than the most recent batch number. Conversely, if you invoke a knex migrate:rollback, it will find the highest batch number (there could be more than 1 migration in a batch...), and invoke the down function on all those files, effectively rollback those changes.
All that said, knex is a "query builder" tool. It's got a ton of helper functions to help build the sql for you. Personally, I find this to be a major distraction. Why spend hours on hours figuring out all the helper functions when I can just go crank out raw SQL and run that. Thus, that's what we've done in our system. we use knex.raw('') and write our own DDL and DML. It works great and does exactly what we need it to. We don't need to go figure out the magic of the query building.
The short answer is that knex will automatically know what has and has not been run for you (again, via that knex_migrations table it creates for you...).
Things can get weird though when it start involving git and different branches. I recommend that if you're writing migrations on some branch, and you need to go do other work, always remember to first perform a rollback of any migrations you've done in that branch BEFORE switching branches. Otherwise you will be in weird DB states that don't coincide with the application code.
I would personally just deal with updating models independently of writing migrations. For example, if I'm adding a description column to some table, then I probably want to manually update the ORM to reflect the change of the new db schema. Generally, I've found trying to use a tool that automagically does that for you (rather, if I change the orm, stuff happens to write all the underlying sql...) usually winds me up in a heap of trouble and I just spend more time trying to un-fudge stuff. But, that's just my 2 cents :)
Here is where it gets a little tough and where I need help...I need to track what has been added or removed via migration so I can effectively write ups and downs as the tableModels change over time.
You could store changes in a DB/txt file and those can act as snapshots. So when you want to rollback to a particular migration, you would find the changes (up/down) made for that mutation and adjust accordingly.
Later I want to add a field called description so I would simply add it to my "tableModel" and then run a build command which would build out the migration. How do I check what has already been created though so I only do an up() for description?
Here you either call the database itself directly and check what fields have already been created. If a field is already their and the attributes are the same, you can either ignore it or stop the transaction all together.
Bonus Round Let's say I want to change user_id from an int to a bigint, because who makes a foreign key just an int? How do I check not just what needs to be added to the up and down, but also checks if I need to change a property on a field.
Again, call the DB itself on the table in question. I know the SQL call would be:
describe [table_name];
After reading the end, I think you answered this yourself, but I think capturing these changes would work best in a NoSql database since you're using Node or PostGres with it's json field.
I have created a non-persistent attribute in my WoActivity table named VDS_COMPLETE. it is a bool that get changed by a checkbox in one of my application.
I am trying to make a automatisation script in Python to change the status of every task a work order that have been check when I save the WorkOrder.
I don't know why it isn't working but I'm pretty sure I'm close to the answer...
Do you have an idea why it isn't working? I know that I have code in comments, I have done a few experimentations...
from psdi.mbo import MboConstants
from psdi.server import MXServer
mxServer = MXServer.getMXServer()
userInfo = mxServer.getUserInfo(user)
mboSet = mxServer.getMboSet("WORKORDER")
#where1 = "wonum = :wonum"
#mboSet .setWhere(where1)
#mboSet.reset()
workorderSet = mboSet.getMbo(0).getMboSet("WOACTIVITY", "STATUS NOT IN ('FERME' , 'ANNULE' , 'COMPLETE' , 'ATTDOC')")
#where2 = "STATUS NOT IN ('FERME' , 'ANNULE' , 'COMPLETE' , 'ATTDOC')"
#workorderSet.setWhere(where2)
if workorderSet.count() > 0:
for x in range(0,workorderSet.count()):
if workorderSet.getString("VDS_COMPLETE") == 1:
workorder = workorderSet.getMbo(x)
workorder.changeStatus("COMPLETE",MXServer.getMXServer().getDate(), u"Script d'automatisation", MboConstants.NOACCESSCHECK)
workorderSet.save()
workorderSet.close()
It looks like your two biggest mistakes here are 1. trying to get your boolean field (VDS_COMPLETE) off the set (meaning off of the collection of records, like the whole table) instead of off of the MBO (meaning an actual record, one entry in the table) and 2. getting your set of data fresh from the database (via that MXServer call) which means using the previously saved data instead of getting your data set from the screen where the pending changes have actually been made (and remember that non-persistent fields do not get saved to the database).
There are some other problems with this script too, like your use of "count()" in your for loop (or even more than once at all) which is an expensive operation, and the way you are currently (though this may be a result of your debugging) not filtering the work order set before grabbing the first work order (meaning you get a random work order from the table) and then doing a dynamic relationship off of that record (instead of using a normal relationship or skipping the relationship altogether and using just a "where" clause), even though that relationship likely already exists.
Here is a Stack Overflow describing in more detail about relationships and "where" clauses in Maximo: Describe relationship in maximo 7.5
This question also has some more information about getting data from the screen versus new from the database: Adding a new row to another table using java in Maximo
I am new to CouchDB / PouchDB and until now I somehow could manage the start of it all. I am using the couchdb-python library to send initial values to my CouchDB before I start the development of the actual application. Here I have one database with templates of the data I want to include and the actual database of all the data I will use in the application.
couch = couchdb.Server()
templates = couch['templates']
couch.delete('data')
data = couch.create('data')
In Python I have a loop in which I send one value after another to CouchDB:
value = templates['Template01']
value.update({ '_id' : 'Some ID' })
value.update({'Other Attribute': 'Some Value'})
...
data.save(value)
It was working fine the whole time, I needed to run this several times as my data had to be adjusted. After I was satisfied with the results I started to create my application in Javascript. Now I synced PouchDB with the data database and it was also working. However, I found out that I needed to change something in the Python code, so I ran the first python script again, but now I get this error:
couchdb.http.ResourceConflict: (u'conflict', u'Document update conflict.')
I tried to destroy() the pouchDB database data and delete the CouchDB database as well. But I still get this error at this part of the code:
data.save(value)
What I also don't understand is, that a few values are actually passed to the database before this error comes. So some values are saved() into the db.
I read it has something to do with the _rev values of the documents, but I cannot get an answer. Hope someone can help here.
I am working on Oracle 10gR2.
And here is my problem -
I have a procedure, lets call it *proc_parent* (inside a package) which is supposed to call another procedure, lets call it *user_creation*. I have to call *user_creation* inside a loop, which is reading some columns from a table - and these column values are passed as parameters to the *user_creation* procedure.
The code is like this:
FOR i IN (SELECT community_id,
password,
username
FROM customer
WHERE community_id IS NOT NULL
AND created_by = 'SRC_GLOB'
)
LOOP
user_creation (i.community_id,i.password,i.username);
END LOOP;
COMMIT;
user_Creation procedure is invoking a web service for some business logic, and then based on the response updates a table.
I need to find a way by which I can use multi-threading here, so that I can run multiple instances of this procedure to speed up things. I know I can use *DBMS_SCHEDULER* and probably *DBMS_ALERT* but I am not able to figure out, how to use them inside a loop.
Can someone guide me in the right direction?
Thanks,
Ankur
what you can do is submit lots of jobs in the same time. See Example 28-2 Creating a Set of Lightweight Jobs in a Single Transaction
This fills a pl/sql table with all jobs you want to submit in one tx, all at the same time. As soon as they are submitted (enabled) they will start running, as many as the system can handle, or as many as are allowed by a resource manager plan.
The overhead that the Lightweight jobs have is very ... minimal/light.
I would like to close this question. DBMS_SCHEDULER as well as DBMS_JOB (though DBMS_SCHEDULER is preferred) can be used inside the loop to submit and execute the job.
For instance, here's a sample code, using DBMS_JOB which can be invoked inside a loop:
...
FOR i IN (SELECT community_id,
password,
username
FROM customer
WHERE community_id IS NOT NULL
AND created_by = 'SRC_GLOB'
)
LOOP
DBMS_JOB.SUBMIT(JOB => jobnum,
WHAT => 'BEGIN user_creation (i.community_id,i.password,i.username); END;'
COMMIT;
END LOOP;
Using a commit after SUBMIT will kick off the job (and hence the procedure) in parallel.