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<concept xml:lang="en" id="GUID-4EE4E79E-F6D5-5F14-BA8D-4DD10D229B74"><title>Handling BLOBs</title><prolog><metadata><keywords/></metadata></prolog><conbody><p>This document introduces you to Binary Large Objects (BLOB) in Symbian SQL. </p> <section><title>Purpose</title> <p>A BLOB is an SQL database container object for binary large objects, usually images or audio files but may also include other binary files such as an application executable. </p> </section> <section><title>Features of SQL BLOB</title> <p>BLOBS offer the following features: </p> <ul><li id="GUID-50CE608F-04BA-5D0F-BA59-E608F4B10989"><p>read from and write to BLOB objects in a RAM-efficient manner -- enables an overall reduction of RAM usage, </p> </li> <li id="GUID-60702664-4ABF-5F90-9DDE-C77969C0DC0B"><p>reduced read latency (compared to previous versions) -- improving the responsiveness of client applications, </p> </li> <li id="GUID-72DAFF78-CF0D-50F0-8C29-BF95920D2CA0"><p>larger BLOB objects can be stored and retrieved (&gt;16Mb). Before version 9.5 there was a limit on the size of a BLOB object supported by Symbian SQL. This limit was dictated by the amount of server-side RAM available. Applications that needed to store large BLOB objects in their database (for example, MP3 files) are no longer restricted by this limit. </p> </li> <li id="GUID-D66B611B-8908-59B1-A7BA-BED5D6702509"><p>The maximum possible length for a blob is 2147483647 bytes. </p> </li> </ul> </section> <section><title>Streaming</title> <p>The classes <xref href="GUID-03426E55-3C0A-322B-9CE7-556D63FD762F.dita"><apiname>RSqlBlobReadStream</apiname></xref> and <xref href="GUID-9D7747B6-E090-3450-9906-845B549F966F.dita"><apiname>RSqlBlobWriteStream</apiname></xref> allow direct access to BLOB content. These classes do not require the whole BLOB to be loaded into RAM before streaming can begin. These classes match the standard Symbian <xref href="GUID-81056EAD-722C-373F-A8CF-4CC67A5B4468.dita"><apiname>RReadStream</apiname></xref> /<xref href="GUID-ADAC48A2-3D24-306B-9473-70201F4D351D.dita"><apiname>RWriteStream</apiname></xref> idiom. </p> <p>The server intelligently chooses the size of an internal buffer to incrementally read/write the requested blob data in blocks. The size of the buffer is calculated based on a combination of the size of the request and the value of <codeph>aBlockSizeHint</codeph> (if specified). </p> <p> <codeph>aBlockSizeHint</codeph> is provided by the client to provide a hint to the server about the size of block that it is intending to use to read/write the blob data. This is advance information about the intended behaviour of the client that the server may use to its benefit to read/write the blob data. </p> </section> <section><title>Whole-blob access</title> <p>The class <xref href="GUID-78544D11-1972-3492-9BD7-D794240895E1.dita"><apiname>TSqlBlob</apiname></xref> allows one-shot storage and retrieval of whole blobs. This class avoids the need for double buffering of the blob in both the client and the server. </p> <p>there are two versions of the BLOB get functions. One is <i>non-leaving</i> and the other is <i>leaving</i>. </p> <p>Use the non-leaving variant when the client knows that the blob will either: </p> <ul><li id="GUID-557D1A55-1E94-56F6-8794-1257ED5D2E17"><p>all be of a fixed size, </p> </li> <li id="GUID-7C5CA733-C030-5761-A2C0-5EAA7A3B50FD"><p>not exceed an upper size limit. </p> </li> </ul> <p>In these situations the client can pre-allocate a buffer and avoid the overhead of memory allocation. </p> <p>Use the leaving variant when the client does not have prior knowledge of the BLOB size (or if the size varies wildly). In this situation the server allocates a buffer and passes ownership to the client. </p> </section> <section><title>RAM benefits</title> <p>Symbian SQL has been improved to eliminate the need for a 2 MB server-side buffer. Data is transferred to the client in blocks so for example, the server-side buffer may be reduced to 32 KB. </p> <p>Previous to version 9.5, both whole-blob and streaming use cases require up to 3MB of server-side RAM. The page cache size is capped at 1MB due to a built-in limiting mechanism. But, a 2MB buffer must be allocated to hold the entire blob. A critical side-effect of this is that, due to heap exhaustion, there is a maximum limit on the size of a blob object that can be written to or read from a database. </p> <p>In the following diagram the top two sections illustrate whole-blob retrieval in which the entire blob is retrieved in one go using <xref href="GUID-0176BF07-DF94-3259-8F90-DE030E35CE9A.dita"><apiname>RSqlStatement</apiname></xref>. The bottom section illustrates streaming retrieval in which the blob is retrieved in blocks specified by the client using <xref href="GUID-35BF7B4A-C4F7-3215-B5DF-6D0682247976.dita"><apiname>RSqlColumnReadStream</apiname></xref>. </p> <fig id="GUID-60AEFF09-235C-55F3-BBEF-D484964A4A82"><title>

             Reading a 2MB blob. 

          </title> <image href="GUID-739556F5-A2F3-5548-943C-7D50785DFB48_d0e360653_href.png" placement="inline"/></fig> </section> </conbody><related-links><link href="GUID-5C3005B3-7008-5209-A2BD-D52B8B422D58.dita"><linktext>SQL Security

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