Storing the Cursor for App Engine Pagination - google-app-engine

Question

I'm trying to implement pagination using App Engine's RPC and GWT (it's an app engine connected project).
How can I pass both the query results and the web-safe cursor object to the GWT client from the RPC?
I've seen examples using a servlet but I want to know how to do it without a servelt.
I've considered caching the cursor on the server using memcache but I'm not sure if that's appropriate or what should be used as the key (session identifier I would assume, but I'm not sure how those are handled on App Engine).
Links to example projects would be fantastic, I've been unable to find any.

Answer 1

OK, so the best way to do this is to store the cursor as a string on the client.
To do this you have to create a wrapper class that is transportable so you can pass back it to the client via RequestFactory that can hold the results list and the cursor string. To do that you create a normal POJO and then a proxy for it.
here's what the code looks like for the POJO:
public class OrganizationResultsWrapper {
public List<Organization> list;
public String webSafeCursorString;
public List<Organization> getList() {
return list;
}
public void setList(List<Organization> list) {
this.list = list;
}
public String getWebSafeCursorString() {
return this.webSafeCursorString;
}
public void setWebSafeCursorString(String webSafeCursorString) {
this.webSafeCursorString = webSafeCursorString;
}
}
for the proxy:
#ProxyFor(OrganizationResultsWrapper.class)
public interface OrganizationResultsWrapperProxy extends ValueProxy{
List<OrganizationProxy> getList();
void setList(List<OrganizationProxy> list);
String getWebSafeCursorString();
void setWebSafeCursorString(String webSafeCursorString);
}
set up your service and requestFactory to use the POJO and proxy respectively
// service class method
#ServiceMethod
public OrganizationResultsWrapper getOrganizations(String webSafeCursorString) {
return dao.getOrganizations(webSafeCursorString);
}
// request factory method
Request<OrganizationResultsWrapperProxy> getOrganizations(String webSafeCursorString);
Then make sure and run the RPC wizard so that your validation process runs otherwise you'll get a request context error on the server.
Here's the implementation in my data access class:
public OrganizationResultsWrapper getOrganizations(String webSafeCursorString) {
List<Organization> list = new ArrayList<Organization>();
OrganizationResultsWrapper resultsWrapper = new OrganizationResultsWrapper();
Query<Organization> query = ofy().load().type(Organization.class).limit(50);
if (webSafeCursorString != null) {
query = query.startAt(Cursor.fromWebSafeString(webSafeCursorString));
}
QueryResultIterator<Organization> iterator = query.iterator();
while (iterator.hasNext()) {
list.add(iterator.next());
}
resultsWrapper.setList(list);
resultsWrapper.setWebSafeCursorString(iterator.getCursor().toWebSafeString());
return resultsWrapper;
}

Answer 2

a second option would be to save the webSafeCursorString in the memcache, as you already mentioned.
my idea looks like this:
the client sends always request like this "getMyObjects(Object... myParams, int maxResults, String clientPaginationString)". the clientPaginationString is uniquely created like shown below
server receives request and looks into the memcache if there is a webSafeCursorString for the key clientPaginationString
if the server finds nothing, he creates the query and save the webSafeCursorString into memcache with the clientPaginationString as the key. -> returns the results
if the server finds the webSafeCursorString he restarts the query with it and returns the results
the problems are how to clean the memcache and how to find a unique clientPaginationString:
a unique clientPaginationString should be the current UserId + the params of the current query + timestemp. this should work just fine!
i really can't think of a easy way how to clean the memcache, however i think we do not have to clean it at all.
we could store all the webSafeCursorStrings and timestemps+params+userid in a WebSafeCursor-Class that contains a map and store all this in the memcache... and clean this Class ones in a while (timestamp older then...).
one improvement i can think of is to save the webSafeCursorString in the memcache with a key that is created on the server (userSessionId + servicename + servicemethodname + params). however, important is that the client sends an information if he is interested in a new query (memcache is overriden) or wants the next pagination results (gets webSafeCursorString from memcache). a reload of the page should work. a second tap in the browser would be a problem i think...
what would you say?