The subject of database locking really is going to start to get too deep for where we're really going in this course, but I've got to mention it, because we've kind of wandered into the woods here about multiple user applications, and so forth, and if you're trying to use Access as a multi-user application, you're going to have to deal with database locking at some point. Now database locking is a functionality that all databases employ, and what we're trying to do here, is prevent multiple simultaneous changes to data. Now the way this happens, is the database really creates one of these take a number scenarios. If you've ever gone into a delicatessen, a pizza place, anything like that, where there's a lot of people, there's a little machine there, you pull a little paper tab off, and it has a number, so you take a number, and when they call your number, then you can step forward and make your order. Same thing happens with the database. It is going to let the first person who gets there to the data, work with that data, and anybody else whose trying to access the data, simply has to take a number and wait, and it'll just you know, the first come first served, and then it goes in order, as they get there. Now locking effects users in a couple of ways. They'll see it as slow response, while they're waiting for another lock to be released, or they may just get an error message, saying, you know, the records locked, and you can't access it. Either way it irritates users. Now, databases in Access employ one of two different methods to apply locks to the data. The first one is a rollover lock, and that is where it locks and individual record or a row, based on certain settings, but the bottom line is, that you have less locks, or less locks happen because Access only locks the rows that I'm working on OK? And so if I'm working on row two, and you're working on row three, there's not going to be a problem there, with me seeing a problem with locks OK? Again, that last locks can be a little confusing there. What I'm saying there is, there will be more physical locks take place, but less lock problems, OK? Because neither one, me or you, working on row three and row four, will see anything about a lock, because each row is locked out individually. Now, page level locking is where data is locked based on the page data that is used to store data in the database. Now what happens is Access stores it's data in 4k chunks, the easiest way to understand this, is if one entire row uses 1k, or 1000 bytes, then I can store four rows on a single data page. Well, guess what? If the first four rows are stored on a data page, and I hit row number three, and it locks it, guess what? I just also locked row number four. You're trying to get on row number four, but when you try to read it, you get notified there's a lock, or you get slow response alright? And so, the end users are probably going to see or feel more locking, but this is more efficient management for Access, so here's where we always end up with database locking, and that is choose your poison OK? We can go to row level locking, where the users will perceive less locks, but Access has to work harder, because it's doing more individual locks. The page level lock will do more locks that the user will perceive, less locks that Access has to manage, and so Access runs a little faster OK? Don't you love being a database administrator by the way. Now let's talk about some default locking options. We can tell Access how we would like it to behave in a number of ways, and let me hop out to the Access environment, and show you here where you do that. If you go to the File Tab in Access, and then come down to Options, if you go to the Client Settings page, and then if you'll scroll down to the Advanced Section right here, you will notice Default Record Locking. No locks, all records, edited records. This is where you can set this. Now don't set anything out here, without reading up on it, making sure you understand what this means, and what's going to happen alright? But that is exactly what we're talking about right here. The No Locks Setting there, means that data is locked only during the rare, very brief period of time, when the data is actually being written to the Access table. Now this is usually less than a second. This is also called optimistic locking, because we're assuming that, that data hasn't changed since between the time that I looked at it, and the time that I'm writing it. Now the user could possibly start an update, and not be able to complete it, because from the time I started to look at the data, to update it, somebody else changed it, or deleted it or something OK? But it's optimistic, we're going to hope for the best. The All Record Setting locks the entire record set, as long as a table form or query is open, and this just simply does not work in a multi-user environment. That means, that if I just double click on a table, and open it up, that table is locked out, until I close it. And obviously that wouldn't work. Then the last option is Edited Record, and this is where the data is locked as soon as the user begins editing a record, so the minute I get on a row, it's going to lock it, this is also called Pessimistic Locking. Two users can edit the same record at the same time, and again, this, this can get confusing with you, but just hang in there with me, OK? Data overwrites can occur without warning alright? So here's the bottom line with this, and this is where all this ends up OK? If you have more than one user in a database, at some point, the database is going to employ locking to keep people from stepping on each other. You can use this No Record, Edited Record, and all this to somewhat turn the knob there, OK? And limit how aggressively it sets it's locks. But you need to read and understand this, but if you get into a multi user environment, and you're having locking problems, this is where you need to go, to start to read up and study OK? So that's a real quick, coverage, on database locking in Access.
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