federation.md
Lucas_Werkmeister_WMDE (Lucas Werkmeister (WMDE))
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	Apr 12 2017, 12:55 PM

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federation.md
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	- I copy the entire MediaWiki installation, wipe the LocalSettings.php and run the setup on the second instance.
	I enable repo and client in the second installation.
	I now have two installations: wiki1 (local) and wiki2 (remote).

	- Now I’m lost for a bit.
	Presumably, the usual consumer of this documentation is assumed to know how to set up regular interwiki links already…

	- I try to set up interwiki links.

	- <https://www.mediawiki.org/wiki/Extension:Interwiki> seems to be outdated?
	I don’t have a Special:Interwiki page,
	but the `wfLoadExtension()` snippet doesn’t work
	(Interwiki.php is in `includes/interwiki`, not in `extensions/Interwiki`).
	- Oh, according to `api.php?action=query&meta=siteinfo&siprop=interwikimap` I already have lots of interwikis set up?
	- Okay, looks like <https://www.mediawiki.org/wiki/Manual:Interwiki> is the more helpful documentation.

	INSERT INTO interwiki (iw_prefix, iw_url, iw_api, iw_wikiid, iw_local, iw_trans) VALUES ('wiki2', 'http://localhost/wiki2/$1', '', '', 1, 0);

	(I leave the `iw_api` and `iw_wikiid` fields blank, which matches most of the existing entries. However, they don’t have a default values, so I do have to set them explicitly.)

	- I create some properties and items in the remote wiki (wiki2).

	- I try to configure the remote repository in the local wiki:

	$wgWBClientSettings['foreignRepositories'] = array(
	'wiki2' => array(
	'supportedEntityTypes' => array( 'item', 'property' ),
	'repoDatabase' => 'wiki2',
	'baseUri' => 'http://localhost/wiki2/'
	)
	);

	The values are mostly guessed –
	I assume that entity types are `item` (type) and not `Item` (namespace);
	that a “symbolic database name” is just a database name;
	that the base URI should point to the remote repository URL;
	and that the `prefixMapping` is optional and defaults to empty.
	(Later, it turns out that this last part produces warnings, so I explicitly specify an empty array.)

	- Upon loading an item in the local wiki, I get a rather ugly error.
	The error message is “(Cannot access the database)”,
	so presumably I need to grant the wiki1 MySQL user access to the wiki2 database
	(I configured the wikis to each create a user for themselves).

	GRANT ALL ON wiki2.* TO wiki1;

	(Presumably, read-only access would suffice, but I’m lazy and also don’t know SQL permissions.)

	- I try to add remote statements.
	At first, I’m confused that there are no suggestions for “remote”
	(I called the sample data on wiki2 “remote property” and “remote item”),
	but entering `wiki2:P3` and `wiki2:Q2` appears to work and shows their labels.
	However, I can’t save the statement: Internal Server Error.
	Dev Tools reveals the error message:

	> Using same entity types on multiple repositories is not supported yet. "item" has already be defined for repository ""

	I guess Commons won’t define items of its own, so that’s why this restrictions makes sense there?

	- Will it work if I just rename the item type?
	Let’s try with a new wiki.

	- I set up `wiki3` just like `wiki2` above.

	- I add this to the local settings (based on `repo/config/Wikibase.example.php`):

	$wgWBRepoSettings['entityNamespaces']['item3'] = WB_NS_ITEM;
	$wgWBRepoSettings['entityNamespaces']['property3'] = WB_NS_PROPERTY;

	- I run the installation scripts as described in the mw:Wikibase/Installation.
	(It’s probably a good thing that I forgot to do this two steps ago and only did it after adjusting `LocalSettings.php`…)

	- I add a property and an item on wiki3.

	- I add the wiki3 interwiki to wiki1 as above (`INSERT INTO interwiki`…).

	- I add wiki3 to wiki1’s `foreignRepositories`, with the `supportedEntityTypes` `item3` and `property3` instead of `item` and `property`.

	- I grant wiki1 access to the wiki3 database, as above (`GRANT`…).

	- I try to open a page in wiki1 and get this error (blank page, error message from Apache `error.log`):

	> PHP Fatal error: Uncaught Error: Call to a member function getCode() on null in /var/www/html/wiki1/includes/user/User.php:1566
	> Stack trace:
	> #0 /var/www/html/wiki1/includes/user/User.php(5231): User::getDefaultOptions()
	> #1 /var/www/html/wiki1/includes/user/User.php(502): User->loadOptions()
	> #2 /var/www/html/wiki1/includes/libs/objectcache/WANObjectCache.php(889): User->{closure}(false, 3600, Array, NULL)
	> #3 /var/www/html/wiki1/includes/libs/objectcache/WANObjectCache.php(1009): WANObjectCache->{closure}(false, 3600, Array, NULL)
	> #4 /var/www/html/wiki1/includes/libs/objectcache/WANObjectCache.php(892): WANObjectCache->doGetWithSetCallback(global:user:id:..., 3600, Object(Closure), Array, NULL)
	> #5 /var/www/html/wiki1/includes/user/User.php(514): WANObjectCache->getWithSetCallback(global:user:id:..., 3600, Object(Closure), Array)
	> #6 /var/www/html/wiki1/includes/user/User.php(445): User->loadFromCache()
	> #7 /var/www/html/wiki1/includes/user/User.php(409): User->loadFromId(0)
	> #8 /var/www/html/wiki1/includes/session/UserInfo.php in /var/www/html/wiki1/includes/user/User.php on line 1566

	Apparently the global value `$wgContLang` is somehow `null`. WTF?

	- However, if I comment out the `wiki2` entry in `foreignRepositories`, it works again.
	I don’t understand why, but I don’t need that part anyways, so whatever.

	- I try to add remote statements and get a different error than before:

	> Could not save due to an error.
	> ⧼wikibase-validator-unsupported-entity-type⧽

	I guess wiki1’s Wikibase is confused by the `item3` type.
	But if I add it to `wiki1`’s `entityNamespaces`, I get the same “using the same entity types on multiple repositories” error as before with `wiki2`.
	Not sure what to do now…

	- Apparently entity types aren’t as free-form as I thought.
	The `ForeignEntityValidator` checks the `$id->getEntityType()` against the supported entity types,
	and an `ItemId`’s entity type is hard-coded to be `'item'`.

	- Let’s try a new version.
	The local repository has no entities of its own at all, only federated ones.

	- I set up `wiki4` (local) and `wiki5` (remote) as before:
	Copy the folder in `/var/www/html`, remove `LocalSettings.php`, run setup, copy over bits of `LocalSettings.php`.

	- I configure the local wiki:

	$wgWBRepoSettings['entityNamespaces'] = [];
	$wgWBClientSettings['foreignRepositories'] = array(
	'wiki5' => array(
	'supportedEntityTypes' => array( 'item', 'property' ),
	'repoDatabase' => 'wiki5',
	'baseUri' => 'http://localhost/wiki5/',
	'prefixMapping' => array()
	)
	);

	- I configure the database for wiki4/wiki5:

	INSERT INTO wiki4.interwiki (iw_prefix, iw_url, iw_api, iw_wikiid, iw_local, iw_trans) VALUES ('wiki5', 'http://localhost/wiki5/$1', '', '', 1, 0);
	GRANT ALL ON wiki5.* TO wiki4;

	- I try to run the installation scripts for wiki4 and wiki5,
	but the very first script for wiki4 already complains that `entityNamespaces` is invalid.
	I change it to:

	$wgWBRepoSettings['entityNamespaces'] = [ 'dummy' => 0 ];

	The scripts print warnings for the “dummy” entry, but seem to succeed.

	- I create the sample property and item in wiki5.

	- I realize that I have a problem –
	where am I going to add statements in wiki4 that refer to wiki5 entities?
	wiki4 has no entities…

	- I try to create an item on wiki4, but get an error that

	> "Wikibase item" content is not allowed on page Q2

	- I decide to allow local items after all, and only use remote properties.
	This doesn’t make any sense with regards to the Commons usecase,
	but I just want to save a federated statement and see that work.

	- I change the wiki4 config again to re-allow items:

	$wgWBRepoSettings['entityNamespaces'] = [ 'item' => WB_NS_ITEM ];
	$wgWBClientSettings['foreignRepositories'] = array(
	'wiki5' => array(
	'supportedEntityTypes' => array( 'property' ),
	'repoDatabase' => 'wiki5',
	'baseUri' => 'http://localhost/wiki5/',
	'prefixMapping' => array()
	)
	);

	- I create a local item on wiki4.

	- I add a statement with the remote property and the local item.
	IT WORKS! 🎆

	- Lydia explains to me how it works on Commons:
	The WikibaseMediaInfo extension adds a new entity type, MediaInfo.
	Commons will have local MediaInfo entities and remote property and item entities.
	Let’s try that out as well, we don’t have enough wikis yet.

	- I set up wiki6 (local) as before (we’ll reuse wiki5 as the remote wiki).
	Copy, remove settings, setup, copy settings.
	I skip the entity namespaces and foreign repositories settings for now.

	- I clone <gerrit:mediawiki/extensions/WikibaseMediaInfo.git> into the extensions/ folder
	and run `composer install` in it.

	- I add the following configuration:

	wfLoadExtension( 'WikibaseMediaInfo' );
	$baseNs = 124;
	define( 'WB_NS_MEDIA', $baseNs + 0 );
	define( 'WB_NS_MEDIA_TALK', $baseNs + 1 );
	$wgExtraNamespaces[WB_NS_MEDIA] = 'Media';
	$wgExtraNamespaces[WB_NS_MEDIA_TALK] = 'Media_talk';
	$wgWBRepoSettings['entityNamespaces'] = [ 'mediainfo' => WB_NS_MEDIA ];
	$wgWBClientSettings['foreignRepositories'] = array(
	'wiki5' => array(
	'supportedEntityTypes' => array( 'item', 'property' ),
	'repoDatabase' => 'wiki5',
	'baseUri' => 'http://localhost/wiki5/',
	'prefixMapping' => array()
	)
	);

	- I configure the database:

	INSERT INTO wiki6.interwiki (iw_prefix, iw_url, iw_api, iw_wikiid, iw_local, iw_trans) VALUES ('wiki5', 'http://localhost/wiki5/$1', '', '', 1, 0);
	GRANT ALL ON wiki5.* TO wiki6;



	- I run the installation scripts.

	- I enable uploads in wiki6 and make its `images/` folder writable to the `www-data` group.

	- I upload a file and follow the MediaInfo link.

	- I add a statement with the remote property and the remote item.
	IT WORKS! 🎆

	- The sitelinks don’t work, by the way – they’re missing the index.php bit
	(I don’t have Apache set up to rewrite that automatically).
	To fix this, I update the interwiki table:

	UPDATE wiki6.interwiki SET iw_url = 'http://localhost/wiki5/index.php/$1' WHERE iw_prefix = 'wiki5';

	And then I run the update scripts.
	(I’m not sure which ones are necessary – apparently both update.php and populateInterwiki.php?)

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