[xwiki-devs] Reduce heap exhaustion during export operation (and partly import for now)

Hi, As you knows, Import/Export operation on large XAR file cause problem. We felt on an even worse situation, where a single XWiki document is not properly exported due to heap exhaustion during the built of its XML DOM or due to large attachments. Having a look at the source, I have notice that many optimizations on the way the export is produce could be quite easily introduced. Currently, XWiki stored the exported document several times in memory during the operation, which impact performance negativly and is uselessly heavy on memory usage, even for reasonable documents. Therefore, I have started a large patch to avoid these caveats, here is the strategies I have followed: 1) The current implementation mostly build a DOM in memory for immediately serialized it into a stream. So I have remove the intermediate DOM and provide direct streaming of Element content by: 1.1) extending org.dom4j.XMLWriter to allow direct streaming of Element content into the output stream, as is, or Base64 encoded. Accessorily, my extension also ensure proper pairing of open/close tag. 1.2) writing a minimal DOMXMLWriter which extends my XMLWriter and could be used with the same toXML() code to build a DOMDocument to provide the toXMLDocument() methods to support the older implementation unchanged if ever needed. 1.3) using the above, minimal change to the current XML code was required 1.3.1) replacing element.add(Element) by either writer.writeElement) or writer.writeOpen(Element) 1.3.2) for large content, use my extensions, either writer.write(Element, InputStream) or writer.writeBase64(Element, InputStream) which use the InputStream for the element content 2) The current implementation for binary data such as attachments and export zip file is mostly based on in memory byte[] passing from function to function while these data initially came from a request.getInputStream() or are written to a response.getOutputStream (). So I have change these to passover the stream instead of the data: 2.1) using IOUtils.copy when required 2.2) using org.apache.commons.codec.binary.Base64OutputStream for base64 encoding when required 2.3) using an extension of ZipInputStream to cope with unexpected close() 2.4) avoid buffer duplication in favor of stream filters 3) Since most oftently used large data came from the database through an attachment content, it would be nice to have these attachment streamed from the database when they are too large. However, I feel that it is still too early to convert our binary into a blob, mainly because HSQLDB and MySQL still does not really support blob, just an emulation. These are also used to be cached in the document cache, and this will require improvement to support blob. However I propose to take the occasion to go in the direction of the blob by: 3.1) deprecating setContent(byte[]) and Byte[] getContent() in favor of newly created setContent(InputStream, int), InputStream getContentInputStream() and getSize() 3.2) Begin to use these new function as much as possible as 2) implied 3.3) this also open the ability to store attachment in another repository that support better the streaming aspect (ie: a filesystem) I am currently testing the above changes onto our client project, and I expect to provide a patch really soon. It will require an upgrade of org.apache.commons.codec to version 1.4, to have access to Base64OutputStream. I feel it will be a first step into the right direction, further improvement should be to: - import XML using a SAXParser without building a DOM in memory - manage JRCS Archive better, the way they are built and store raise the same issue then attachment - manage the recycle bin better for the same reason - improve caching to avoid caching very large stuffs WDYT ? Denis Gervalle -- SOFTEC sa http://www.softec.st