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James Rose

c# Short URL from GUID

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Hello gang,

 

I am working on a new project for a site and I would like to implement short urls.  Historically I have used GUIDs as table ids so that replication is not an issue.  So, now I'm looking at creating a short url for these values, the thing is a shortened guid is not that short (vs a shortened Int) Before getting too far down a path, I thought I'd ask if anyone had any thoughts.

 

Thanks

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What is it you are trying to do?

Like you have something say:

guasd3 = www.google.ca
-3idjqis = www.neowin.net

or?  

I guess I don't understand what you are trying to do exactly.

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Just generate strings of whatever size you consider to be short, made up of numbers, uppercase letters and lowercase letters, and associated them with your ID field.

 

[short_url_assoc_table]

table_id : GUID

short_url : string

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What is it you are trying to do?

Like you have something say:

guasd3 = www.google.ca

-3idjqis = www.neowin.net

or?  

I guess I don't understand what you are trying to do exactly.

 

 

I need to use GUIDs as the table ident fields for the issue of replication.  If I use numeric values for the identity, which is easy to convert to a short URL by using Base64 (quite a number of examples around the net for doing this)  however if I use numeric I'm going to have collisions when multiple machines are making new records.  When I have looked at making a short url from a guid, the value is shorter than the GUID, but still longer than the average short GUID.

 

Just generate strings of whatever size you consider to be short, made up of numbers, uppercase letters and lowercase letters, and associated them with your ID field.

 

[short_url_assoc_table]

table_id : GUID

short_url : string

 

Interesting idea, but this could be an issue with replication.  I am also concerned about multiple identity fields (waste of time, space, etc)   Thanks though

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Interesting idea, but this could be an issue with replication.  I am also concerned about multiple identity fields (waste of time, space, etc)   Thanks though

 

What exactly is being replicated?

 

 

I need to use GUIDs as the table ident fields for the issue of replication.  If I use numeric values for the identity, which is easy to convert to a short URL by using Base64 (quite a number of examples around the net for doing this)  however if I use numeric I'm going to have collisions when multiple machines are making new records.  When I have looked at making a short url from a guid, the value is shorter than the GUID, but still longer than the average short GUID.

What am I missing here? This shouldn't be an issue with a relational database. You don't tell your database what the ID of a row is, you let the database decide when it inserts the row(s).

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Im not sure where replication is happening.  Also why do you have to use guids for the identity, why not just use an incrementing number, and hash it's value or something to get the url you want to use?  Also, the database should be able to handle the ID itself.. and there should never be an issue of too many inserts causing problems.

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Just throwing a wild idea but wouldn't a CRC of the Guid as a string will give you a nice short form? I am sure a CRC might repeat at some point but all you have to do is add some more text to it.

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Okay gang,

 

Replication of tables between multiple servers cannot use numeric values as, for example in SQL Server the Identity value is incrimented by 1 (and yes, you can change this value, but it wouldn't help when the app needs to scale)  Imagine two servers, each one adding new values to a table; "Customers" One each server they would both get identity #1 for the first record, when the two servers attempt to merge (ever hour, every minute, whenever) there would be a collision since there would be two records with the same identity value.  Using GUIDs for the key field avoids this issue as it is, almost, impossible to have the same guid twice.

 

 

Thanks Asik, however the "guidAsString" variable is still too long to be a short url.

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Looks like I found the answer.

                //Guid guid = Guid.NewGuid();
                string sGUID = "a33d4a21-7d95-41f7-859e-bf02b2fda650"
                string hashCode = String.Format("{0:X}", sGUID.GetHashCode());
                Console.WriteLine(hashCode);

This makes a nice small url, can anyone think of why this should not be used?

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^you shouldn't use it for the reasons listed here (about uniqueness guarantees and differences between versions): http://stackoverflow.com/questions/7458139/net-is-type-gethashcode-guaranteed-to-be-unique

 

Hash the GUID using SHA1 and truncate it or something like that. That's probably the best you are going to do. (perhaps you will have to truncate it to much, forcing a too high of probability for collision -- you should check the probability).

 

EDIT: Oh also, if you encode the result of hashing in a higher base, you reduce the amount of information loss during truncation.

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^you shouldn't use it for the reasons listed here (about uniqueness guarantees and differences between versions): http://stackoverflow.com/questions/7458139/net-is-type-gethashcode-guaranteed-to-be-unique

 

Hash the GUID using SHA1 and truncate it or something like that. That's probably the best you are going to do. (perhaps you will have to truncate it to much, forcing a too high of probability for collision -- you should check the probability).

 

EDIT: Oh also, if you encode the result of hashing in a higher base, you reduce the amount of information loss during truncation.

 

EDIT:  Maybe what the article is saying, and what you are trying to tell me is that two different GUIDs could return the same hex?

 

 

 

Pardon me if I appear dense; I just read that article and ran some test against the same guid value for 1 billion iterations and it always comes up with the same hex.  I understand that 1 billion isn't necessarily that large a number...  what I am asking is shouldn't the hex value for a specific string always return the same hex value. 

 

Quote: "does not guarantee unique return values for different objects."  Since the app will pull the guid from the db, and then issue a hex on demand wouldn't that value always be the same?

 

Thanks for your input

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Pardon me if I appear dense; I just read that article and ran some test against the same guid value for 1 billion iterations and it always comes up with the same hex.  I understand that 1 billion isn't necessarily that large a number...  what I am asking is shouldn't the hex value for a specific string always return the same hex value. 

 

Quote: "does not guarantee unique return values for different objects."  Since the app will pull the guid from the db, and then issue a hex on demand wouldn't that value always be the same?

 

Thanks for your input

 

It's the same each time because you are using the same version of the .net runtime on the same object for each run so it's producing the same hash. What they are saying is really two things: (1) if you switch versions of the .net runtime (e.g. 3.5 to 4), the returned result can be different for the same object, and (2) and within the same version of the runtime (e.g. 4) there can be collisions in hashes between different objects. There are no uniqueness guarantees.

 

So for example GUID_A.getHashCode() can return different results if you switch .net runtimes. And GUID_B.getHashCode() and GUID_C.getHashCode() could return the same result in the same runtime.

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So for example GUID_A.getHashCode() can return different results if you switch .net runtimes. And GUID_B.getHashCode() and GUID_C.getHashCode() could return the same result in the same runtime.

 

yea, this is the answer I finally got to (see my edit above).  I was having a very hard time getting to the idea that a 30+ char piece of data could reliably be set to a shorter value.

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yea, this is the answer I finally got to (see my edit above).  I was having a very hard time getting to the idea that a 30+ char piece of data could reliably be set to a shorter value.

Well in any case, you should re-encode whatever you do use to a higher number base that is still valid as url characters. For example, as I was saying before if you do the following you can store more information of your hash in less characters. 

String result=re_encode_as_base_X(SHA1_hash(GUID), N) //base 16 --> base N

I think at the end of the day, you will have to truncate though regardless of what you do. 

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It turns out I may be suffering from "doing this too long" desease.  Someone was kind enough to send a private message to me that the issue of replication on numeric  idenities may no longer be the issue it used to be.

 

I'm reading this article now:  http://technet.microsoft.com/en-us/library/ms146907%28v=sql.105%29.aspx

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Thanks Asik, however the "guidAsString" variable is still too long to be a short url.

I was suggesting taking the BigInteger and passing it through whatever method you mentionned that converted numerical values into short URLs, not taking it as a string directly. Anyway, looks like you found your answer.

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